As far as I know there isn't such a thing, there are companies like Rogz or Lupine that will guarantee the collars but none that can't be destroyed, except of course for choke chains or metal prong collars.

The only thing I can suggest is for the dogs not to wear collars when they are playing with each other, that way they can't chew on each others collars. Besides it's actually safer for them not to play with a collar on because there is the odd chance that a jaw or some other body part could be caught in the other dogs collar while playing which could hurt one if not both dogs. That's one of the reasons that dogs at my work have all collars and harnesses removed before entering a play room.