I do tests by 80 lvl rogue with Mutilate, 2 equipped 50641 daggers (326-490 damage) with same poisons at both to
50 level creature
1. Item 43233 applied poison
Poison detected by Mutilate code for mainhand part:
fixed_bonus = 44
normalized weapon damage (min = 446,8 max = 610,8) -> 543
total bonus spell damage (except non-weapon effects spell damages ==0) = 587
*1.2 = 704;
after MeleeDamageBonusDone -> 704
after MeleeDamageBonusTaken -> 704
after CalcArmorReducedDamage -> 592
after CalculateDamageAbsorbAndResist -> 592
final = 592
Poison detected by Mutilate code for offhand part:
fixed_bonus = 44 (0.5 offhand) = 22
normalized weapon damage (min = 223.4 max = 305.4) -> 292
total bonus spell damage (except non-weapon effects spell damages ==0) = 314
*1.2 = 376
after MeleeDamageBonusDone -> 376
after MeleeDamageBonusTaken -> 376
after CalcArmorReducedDamage -> 316
after CalculateDamageAbsorbAndResist -> 316
final = 316
2. Item 43235 applied poison
Poison detected by Mutilate code for mainhand part:
final = 578
Poison detected by Mutilate code for offhand part:
final = 267
3. Item 3775 applied poison
Poison detected by Mutilate code for mainhand part:
final = 656
Poison detected by Mutilate code for offhand part:
final = 214
4. Item 6947 applied instant poison with talent rank 2
Poison detected by Mutilate code for mainhand part:
final = 582
Poison detected by Mutilate code for offhand part:
final = 564 (crit, 25 blocked)
So i see all cases poison detected and 1.2 coef applied. I only not sure is in case offhand attack must applied 0.5 coef to damage as offhand weapon use.