Well, they are the same skill but different specialties. I, too, have done both jobs and I find two reasons to not have 1 person do both jobs… first is the weight problem. Humping a pig and 300 rounds on a forced march is bad enough when you have an assistant to carry the spare barrel, tripod and the other 600+ rounds of ammo. And people tell me now I’m crazy for carrying my M14 and 10 loaded mags. lol. But then, on top of the 50 pounds, you want to add either an M79 or M203 and several pounds of grenades… and that’s just a patrol weight, not sustainment.
My other reason is that now you’ve put all your eggs in one basket. One well-placed RPG or even one stray bullet and now you’ve lost both your gunner and your grenadier. If you want to be trained so you can be assigned either, that is fine. But you’ll have to pick which one you’ll carry.