Are you fostering for a rescue? Can you explain how this foster thing came about? I mean, if you are having a problem between two puppies, maybe the best thing to do, is to call whoever you are fostering for and get some advice.
If that is not an option, what I would do is set up two crates. Food and high value items like rawhides are only given when they are both in their crates. If toys are a problem, play with them individually.
Two puppies at once is hard a lot of the time, perhaps more often than not.
Is your boy going to training classes? Up the training.
RIP Arwen, CD RN CGC
RIP Whitney, RN CGC
Jenna, RN CGC & Babs, CD RA CGC HIC (not AKC)
Heidi, RA CGC & Tori, RN CGC
SG3 Odessa, SchH1, Kkl1, AD
Ninja, RN CGC & Milla, RN CGC
Joy, Star Puppy, RN CGC
Dolly CGC & Bear