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.
Jenna, RN CGC & Babs, CD RA CGC HIC
Heidi, RA CGC
SG3 Odessa, SchH1, Kkl1, AD
Ninja, RN CGC & Milla, RN CGC
Joy, Star Puppy, RN CGC
Dolly CGC & Bear CGC
Hepzibah & Hannah