My pawn was in combat against a tribal armed with a recurve bow. My charge rifle shot shoots off the right arm (i.e. destroyed) but the tribal was not only able to complete the bow shot it had started but also was able to fire again.
There is no "2 handed weapon" stat or similar in Rimworld. Losing one arm simply means that they lose 50% manipulation, which reduces accuracy. All health stats are used for formulas for different generalised actions and nothing overly unique for special subtypes of actions. You can check a pawns ranged accuracy stat to see the effect.
The 50% accuracy malus is caused because they are using their teeth to draw back the bow. Sometimes realism gets sacrificed, I get it though, it seems odd.