The best way to do it is to buy a second cage. Put the two cages in the same room, then move them a little closer together every day. When they seem to be okay being near each other, let them out to play together - make sure you supervise them! Eventually, you'll be able to keep them in the same cage.
Does the chinchilla who isn't eating much have any other symptoms? If she does, it may be time for a vet visit.