Metroconizole or metronidazole? The first is used to treat yeast infections, the second is often used to treat the underlying cause of diarrhea... So I'm thinking maybe it was metronidazole?
At this point I think I'd bring in stool samples to have your vet run a fecal analysis. If your other dog is also getting diarrhea and nothing has changed (they're eating the same food as always, no excessively stressful occurrences like a death in the family or a move, etc), then likely they're picking something up from the environment. There are a number of different things it could be and not all are treatable with metronidazole--but a fecal analysis is probably going to discover what organism is causing the problem.
Keep a close eye--if it's something like campylobacter it can get pretty severe. Excessive diarrhea can cause dehydration--especially if one or the other develops vomiting, too. We've had dogs that needed overnight IV rehydration at the vet clinic while fighting campylobacter.
I hope your beagles are better soon!
Keep us posted, pls.