Some cats just prefer to graze over the course of the day instead of eating everything in one meal. The other cat may be eating more than his / her fair share and the fat one isn't getting enough. It wold be best to feed the cats with separate bowls and feed enough food to each cat so they maintain weight, or in the case of the fat one, lose weight.
What brand of dry food are you feeding the cats? Can you feed canned food as well?
What brand of dry food are you feeding the cats? Can you feed canned food as well?
