Dave, I would guess that they would take both guns since they have no real way of knowing whether or not you fired one or both until they get some witness statements. Also, at that point you are a suspect, so I would expect them to take any guns, knives, etc that you have on you that could be used as a weapon. Once they get some witness statements and have some time to sort things out, it is possible that they might a return a gun that they are certain wasn't used in the shooting and has no value as evidence.