POE2 Giants blood

Either GGG math is wrong or the tooltip that displays damage is wrong. I've looked into Giants blood and dualwielding heavy weapons and the damage in the tooltip is not right. Whatever I try i get significantly less damage than i'm supposed to.

Soo Giants Blood in itself does not reduce damage, but most skills have a 30% reduction in damage (example leap slam). It says 30% less damage, so I assume its multiplicative:
X = damage weapon 1
Y = damage weapon 2, this can also be expressed as k*x where k is an unknown constant. If we want to find the breakpoint, i.e. at what k the damage is unchanged, we do one-handed = two-handed =>

x = (x + kx)*0.7

Since there is an x in each term, we can simply remove it:

1 = (1 + k)*0.7

shuffling around we get k = 0.43

Meaning if weapon 2 is has above 43% of the damage of weapon 1, the total damage should increase. However, my damage consistently decreases, unless my second weapon is significantly more powerful than weapon 1

I'm I missing something?