I have two young birds, and one of them won't eat, what do I do?

Considering how new they are, Moxxy may just be a little stressed, although the fact that he is trying to eat is a good sign because it shows it isn't illness. You probably don't have to take them back to the store since that really has nothing to do with it. The call to the pet store and Moxxy eating are probably just coincidences. Separating them is a good idea, but I would just try to put two separate food bowls into the cage. Pebbles has obviously picked the main bowl as "hers" and the toy is Moxxy's. Try separating the food into two bowls. If Pebbles decides to claim this one and/or Moxxy still isn't eating, then separating should be tried.