I found a way around this, when ammo drops from enemies and you intend to switch weapons leave the ammo there. Switch to weapon you want then pick up ammo. Down side is you may not have as much ammo but by doing this you can switch to second weapons of same class. This really works well in boss fights or where enemies are many. During beta did this many times switching from sniper rifle to fusion rifle. Just remember not to pick up the ammo until you have the weapon you intend to use equipped.