In all honesty, I wouldn't even consider it...not if there is a possibility of bringing more of a risk/danger on your son. Although your dog doesn't nip your older son, there is still the danger for your son. Yes, little nips here and there are a "natural" thing for young puppies during play and whatnot...but are you letting him get away with this behavior? I'd be very strict about a no-biting, no-nipping "policy" for the safety of you and yours. If you let him get away with this now, you are going to be deep in it when he gets much bigger and is already set in his ways.

Yes, the two puppies could just direct more of their nipping, playful energy toward one another...but what happens if they don't? There is no guarantee...

As far as training 2 puppies go, it depends on the person. I, for one, wouldn't be able to do it...but that's just me. If you have the time, patience, and ability to spread your time evenly over both of them then think about it. Again, it's all up to you and your judgement.