2 Scepter Weapon Sets But Only One Allows Allocation of Spirit

I have a scepter in set 1 and set 2. They are different scepters but have the same amount of spirit. Yet I am not able to allocate minions to both sets (e.g., two arsonists can be active for set 1 but not set 2 or vice-versa, the game citing not enough spirit for the other set).