Help!

So you guys probably already saw my two other threads in which I mention my new axie, Marvin, isn't pooping.

I am just the type of person to go overboard when it comes to worrying about my pets (trust me my fiance is incredibly annoyed with me). I am confident that Medusa is pooping. I have found poop every couple days in the tank but I am NOT confident that Marvin is pooping. They both are eating Hikari sinking carnivore pellets every day as well as getting blood worms and mysis shrimp once a week or so. I am trying to get them to switch to night crawlers but so far they're just regurgitating even the smallest blanched pieces...

Anyway... I was wondering if anyone can tell me if there are any signs to look for to tell for sure if he is pooping or not? He occasionally floats his tail end but not his whole body. He does seem slightly more buoyant than he should... but other than that he's eating and behaving normal. He doesn't look bloated to me but I only just got them on October 9th so I could be wrong there.

Are they in the same tank? How do you know which axolotl the poop is from?