Your first problem is that Mom can't feed 2 babies with one nipple. This is why she will feed one and not the other. Eventually the kits will either fight for the one good nipple which could cause fatal injury, or one will get weaker and weaker from lack of food and will die.
To keep this from happening you should begin hand feeding. I would suggest feeding them both. Its quite possible that mom didn't get enough milk which would explain why they chewed her up. If that is the case you should assume that the milk from that one nipple may not be enough to feed either baby.
Keep a close eye on the other nipples because babies have been known to chew the nipples right off. Since they are already scabbed over you may not can see the full damage done. Infection could set in which would call for antibiotics. Just in case your vet doesn't know a lot about chinchillas, Baytril is the antibiotic of choice.
Are you a Chinchilla Club or Chinchilla Breeders Org member? If so, there is an article about a female getting chewed up by the babies here:
http://www.chinchillaclub.com/members/mag/0803teat.htmlIts in the Back issues of the Chinchilla Community Magazine so I'm afraid you won't be able to access it if you aren't a member.