There are 2 types of TWW: Ballistic and Melee
Neither are needed to actually attack with both hands (as per errata of DH)
The armory section for grenades states they use ballistic skill. So if you actually had TWW (ballistic) you don't think that would apply to grenades because they are thrown (I mean they even have melee for the other type, and you use your "weapon" skill for that)? I assume you do the same for shurkin/thowing knives where you don't even allow them to throw 2 in the same turn under any circumstances then since they are thrown (which is counter to the errata as I read it)?
From DH errata 3.0:
"If you do not have the Two Weapon Wielder talent and you wield
two weapons at the same time, you can use a Full Action to make a
single attack with each weapon. However, you suffer a –20 to the
attack roll with the weapon in your dominant hand and –40 with
the weapon in your off hand."