Do weapon damage skills use dual wield or main hand only?

[sotanaht]

For skills that base their damage off of your weapons and do not specify a weapon type, such as Swallow, do they use the combined/averaged damage of both weapons or only the main hand weapon? If both, do shields count as a weapon in this calculation? Ammo?

[HousePet]

Those talents typically do a normal bump attack using the multiplier and damage type specified.