I've always brumated all my snakes together regardless of species. Rubber boas, sirtalis (concinnus), and ordinoides together. Of course these were snakes that den together in the wild anyway, and won't dine on each other. But I always separated them into breeding pairs or by species before slowly warming them up in the spring. The thing about rubber boas though, is that they are most active in the dark/twighlight with a temperature in the low to mid 50's. In order to truly brumate those, they had to be kept colder than I would keep my garters. Still, I did keep them together during more than one winter at 45-55 degrees. They didn't seem to mind and I found no problems with doing so.