It could not hurt to take him to the vet. I take all new animals to the vet for a check up. Runny stool can indicate a possible parasite problem. I think it would be a good idea to take both frogs to the vet. Have him do a fecal test and treat with metronidazole and few days later fenbendazole. Treat both frogs to be on the safe side.
Petco has a tendency not to clean cages in between new inhabitants and house things together that should never be kept together. So, even if they are captive bred there is a possibility it may have picked something up there.