Using One-Handed Melee WeaponsA character may choose to use a one-handed weapon (Reach 0or 1) with both hands to swing with more strength and power. Thisadds a +1 DV bonus to such attacks. Only weapons capable of beinggrasped with two hands may be used in this manner; knives andsimilar small weapons do not get any bonus when used in this way.
Using Two-Handed Melee Weapons with one handA number of melee weapons are designed to be used withtwo hands. These include most pole arms and most weapons witha Reach of 2, plus any others the gamemaster designates as requiringtwo hands. It is possible for a character to use a two-handed weaponwith only one hand, though doing so is difficult and the attack isless powerful. Apply a –2 dice pool modifier to such attacks andreduce the DV of the attack by –2.Trolls can wield such two-handed weapons in a single handmore easily, suffering only a –1 dice pool modifier and a –1 to DV.
While using two hands rather than one might increase damage in some situations (like when you plunge a knife into a prone target - sacrifice style), most melee weapons designed to use with one hand are often harder / slower to control if you wield them with two hands (just imagine a knife fight where one of the combatants swings his knife while holding it with two hands...)I would give +1 recoil compensation if you hold a one handed firearm (like a pistol) with two hands rather than one (this would be a house rule).
You are thinking of Optional Advanced Combat rules from Arsenal, specifically pg 161QuoteUsing One-Handed Melee WeaponsA character may choose to use a one-handed weapon (Reach 0or 1) with both hands to swing with more strength and power. Thisadds a +1 DV bonus to such attacks. Only weapons capable of beinggrasped with two hands may be used in this manner; knives andsimilar small weapons do not get any bonus when used in this way.Same page also covers the opposite, just in case that was your next question:QuoteUsing Two-Handed Melee Weapons with one handA number of melee weapons are designed to be used withtwo hands. These include most pole arms and most weapons witha Reach of 2, plus any others the gamemaster designates as requiringtwo hands. It is possible for a character to use a two-handed weaponwith only one hand, though doing so is difficult and the attack isless powerful. Apply a –2 dice pool modifier to such attacks andreduce the DV of the attack by –2.Trolls can wield such two-handed weapons in a single handmore easily, suffering only a –1 dice pool modifier and a –1 to DV.To date, do not recall anything similar in 5th, but may have missed something in one of the extras.
Hmm, that's true. Which leads us to a different topic...