Because mutilate attacks with both weapons, both attacks have a chance to crit. The crit chance for an additional combo point is 2 x Crit - Crit2.
The formula to calculate Mutilate damage is (corrected regarding offhand damage, see note at end):
Offhand Reduction = 0.5
Average Weapon Damage = (Weapon Minimum Damage + Weapon Maximum Damage) / 2
Modifier = (Attack Power / 14) * 1.7
Base Damage = Average Weapon Damage + Modifier
Mainhand Base Damage = Base Damage + Mutilate Bonus Damage
Mainhand Damage = Mainhand Base Damage
Mainhand Critical Damage = Mainhand Damage * 2
Offhand Base Damage = (Base Damage * Offhand Reduction) + (Mutilate Bonus Damage)
Offhand Damage = Offhand Base Damage
Offhand Critical Damage = Offhand Damage * 2
Minimum Mutilate Damage = Mainhand Damage + Offhand Damage
Maximum Mutilate Damage = Mainhand Critical Damage + Offhand Critical Damage
The formula above has been changed to reflect what actually happens regarding offhand damage. Previously, the Mutilate Bonus Damage was listed as being reduced by the offhand penalty. However, it is in fact not reduced by the offhand penalty.