First, have 2 dual-wieldable weapons (a Needler and an SMG for example), but don't have them out at the same time. The Needler being the one you want extra ammo with, have the SMG out and find another Needler to dual wield with it. Then, find yet another Needler and trade that with your SMG. You'll have 3 Needlers, and have 3x the ammo.