Hex and Mark interactions with mercenary

My assassin character is using a sniper mercenary that has poacher's mark as one of her listed skills. The assassin has a level 20 assassin's mark with mark on hit. If we're both attacking a target, I clearly see that my assassin mark (blue cross thing) overrides her poacher's mark (yellow/orange icon).

The question is when I anoint her amulet with "whispers of doom" for +1 additional curse and then give her a "curse vulnerability on hit", does her vulnerability application stack with my assasin mark application on the same target (rare or unique enemy)?

I looked through the wiki about curses and marks and it states that with +1 curse, the same enemy can have both 1 hex and 1 mark. Does this rule also apply when my sniper is curse hitting with vulnerability and I am doing "mark on hit" with assasin's mark? I myself still have a curse limit of 1. When she applies vulnerability to a rare/unique target, and then I attack it to trigger "mark on hit", I see the assasin mark icon appear on the target and the vulnerability icon disappear. I just want to be sure that I am receiving both the benefits of her vulnerability and my mark on the same target.

Mark on hit has a cooldown, so you'll have decent vulnerability uptime (you apply mark with limit of 1, then merc applies his mark and hex with limit 2 -> 4 sec cooldown -> repeat).
For marks, hovewer, only one of them can be on a target at any moment. So you'll have to either drop this merc, drop assassin's mark, or get used to alternation of marks.

No, that was not my point of confusion.

My "mark on hit" application of assasin's mark immediately overwrites her poacher's mark application, and I no longer see the poacher's mark icon on the target until it dies, or I mark a different target. This behavior agrees with the wiki description of MANUALLY casted debuffs from a mercenary NOT overwriting the player's debuffs on a target. This part works fine I think.

My question is if her "on hit" applied vulnerability (from a ring, not one of her skills) can be on the same target as my assasin mark if she has a "whisper of doom" (+1 curse) anoint.

I ran an eater of worlds invitation last night and he was alive long enough to see rotating assasin mark and vulnerability icons above his head at the same time. I couldn't tell if the vulnerability icon was flickering or a constant 100% uptime but he was definitely dying way faster than usual, even though "on hit" vulnerability is a level 1 hex.

Target can only have 1 mark on it, regardless of curse limits.

So if you apply your mark it will override the Mercs mark and vice-versa.

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My question is if her "on hit" applied vulnerability (from a ring, not one of her skills) can be on the same target as my assasin mark if she has a "whisper of doom" (+1 curse) anoint.

Yes. It should work, however the order matters, because of your curse limit. If you ass-mark, your Merc will be able to apply vuln as well. But if the vuln is on first, your ass-mark will override it, since your curse limit is 1, then the Merc will need to apply Vuln again to have both on.

I think if the player with the less curse limit applies a curse, it will override the number of curses down to the casting players curse limit, in priority order. (I am not certain of this, someone more familiar with party play can confirm.) You should also be able to test it. If the enemy has poachers mark and vuln from you merc and you apply ass-mark, is the vuln also being removed?

Therefore, in your case, if you are frequently applying your ass-mark through mark on hit, it may be that it is not worth it, if the Merc isn't frequently reapplying vuln after you. It might still be useful if you are getting any uptime on the 2 curses, depending on the Merc's behavior and the cooldowns. Or for normal/magic enemies.

The sniper starts shooting far away so her poacher's mark usually gets to a rare/unique target before i start doing my reave (melee ability) attack. As soon I start attacking my mark on hit assasin mark immediately overrides her poacher's mark and i never see the icon again unless i switch to marking a different target.

You mentioned party play and that really is the source of the question here. The game treats the mercenary as an "ally" similar to another party member, but with less monster life and reward scaling than a full additional party member.

I haven't done much party play myself but I assume that it's fine if two different curses, or one mark and one hex, but NOT two marks, are applied from two different party members to the same target? If that's fine with each party member having a default curse limit of 1, then this mercenary situation of mine should be fine.

Like i said in the eater fight I saw revolving assasin mark and vulnerability icons above his head during the phase where i can damage him. I couldn't tell if either of the icons were flickering suggesting a loss of uptime. I never saw the poacher mark icon his head again except for the first 1-2 seconds of a phase where the mercenary shoots him before I can attack my first time.

The wiki part about mercenaries states:

"Mercenaries avoid using curse skills on enemies that are already cursed, even if they have a high enough curse limit to be able to apply the curse without overwriting existing curses."

If this relates to marks, then the sniper should not try to apply poacher's mark on a target once the game sees that my assasin mark is applied. Maybe she can still apply her vulnerability on hit curse even without the +1 curse anoint then?

Edit: Okay just did some further testing on map bosses with and without her +1 curse anoint.

Without the +1 curse anoint, so we each have a curse limit of 1:

*She manually applies poacher mark on target
*She shoots the target and immediately shows vulnerability with poacher mark disappearing, consistent with a curse limit of 1
*I attack the target and see assasin's mark, but can't tell if the vulnerability icon is still there
*I stop attacking, she attacks again, and my assasin's mark is removed on the target and replaced by vulnerability

With the +1 curse anoint, so I have curse limit of 1 and she has 2:

*She manually applies poacher mark on target
*She shoots the target and the target now shows both poacher's mark and vulnerability, consistent with curse limit of 2
*I attack the target just enough to apply assasin mark
*I completely stop attacking and run around while she keeps using attack skills on the boss
*During this time I keep seeing simultaneous assasin mark and vulnerability icons on the boss's head

Overall, I'm confused as to why her curse limit of 1 would remove my assasin's mark from the target, unless she keeps trying to apply her poacher's mark, which is then overwritten by her vulnerability, but that would also screw up the test with a curse limit of 2, and the wiki states that she shouldn't try overwriting my mark with her mark. Anyways, the strategy still works if she has a curse limit of 2+ and the single target damage difference is big.

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I haven't done much party play myself but I assume that it's fine if two different curses, or one mark and one hex, but NOT two marks, are applied from two different party members to the same target? If that's fine with each party member having a default curse limit of 1, then this mercenary situation of mine should be fine.

No, I don't believe so. Curse limit does not stack in the party, so if they both have curse limit of one they will override each others curse.

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Like i said in the eater fight I saw revolving assasin mark and vulnerability icons above his head during the phase where i can damage him. I couldn't tell if either of the icons were flickering suggesting a loss of uptime. I never saw the poacher mark icon his head again except for the first 1-2 seconds of a phase where the mercenary shoots him before I can attack my first time.

Yeah sounds right. I'm not sure how the Merc's are coded, but if the mark skill they are using is self cast, it is possible they only apply their mark if there is not mark already applied, such as new phase before you have applied ass-mark.

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The wiki part about mercenaries states:

"Mercenaries avoid using curse skills on enemies that are already cursed, even if they have a high enough curse limit to be able to apply the curse without overwriting existing curses."

Interesting, I did not know that. Good to know, explains the above.

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If this relates to marks, then the sniper should not try to apply poacher's mark on a target once the game sees that my assasin mark is applied. Maybe she can still apply her vulnerability on hit curse even without the +1 curse anoint then?

I think you need the + curse limit on her. What you are saying is correct I think, she will apply the vuln curse on hit because it is automatic, but without the extra curse it will override your ass-mark. Mark is counted in the curse limit, someone needs 2 curses to have both of them on. The one you dont really seem to need is the poachers mark, though if there is 2 targets, it might be possible to have one on each if you are casting them separately. Again, also not familiar with the nuance of party play, so that may not be possible.

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Without the +1 curse anoint, so we each have a curse limit of 1:

*She manually applies poacher mark on target
*She shoots the target and immediately shows vulnerability with poacher mark disappearing, consistent with a curse limit of 1
*I attack the target and see assasin's mark, but can't tell if the vulnerability icon is still there
*I stop attacking, she attacks again, and my assasin's mark is removed on the target and replaced by vulnerability

With the +1 curse anoint, so I have curse limit of 1 and she has 2:

*She manually applies poacher mark on target
*She shoots the target and the target now shows both poacher's mark and vulnerability, consistent with curse limit of 2
*I attack the target just enough to apply assasin mark
*I completely stop attacking and run around while she keeps using attack skills on the boss
*During this time I keep seeing simultaneous assasin mark and vulnerability icons on the boss's head

Yep that all checks out. It's possible there is some down time on vuln between you applying ass-mark and her hitting again, but it sounds like this is working as you want it to, with either good or full uptime on both ass-mark and vuln.

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Overall, I'm confused as to why her curse limit of 1 would remove my assasin's mark from the target, unless she keeps trying to apply her poacher's mark, which is then overwritten by her vulnerability

Since marks are counted in curse limit and curse limit does not stack between party members. So your ass-mark is on, when she hits, the vuln on hit replaces it, then when you hit, if your mark on hit is off cooldown, it will replace the vuln, and so on.