Pictures of the belly would help us in determining what is wrong with the belly. Most of us on here do not put more than one ball python per enclosure. This is because it can stress out the snakes. Which may be the reason that they are not eating. Another issue with having more than one snake per enclosure is that if one is sick both can end up sick.

If you can provide more information on your setup would be appreciated. Please include temps, humidity, hides, and enclosure size.