You should be able to carry two primary weapons. There is no physical reason why a soldier couldnt carry two primary weapons. There can be a detriment such as stamina being eaten away quicker but for sake of realism it should be an option. Also, there are many times when a friendly could use an extra gun and the only way to get him one is to give away your own primary. It would be a big help to just give him the one strung on your back.