Males can be done as soon as the bits drop. This usually happens around the 3-4 month mark. When I got George done it cost £45 standard price but different vets will vary. They should be fine together as long as they aren't separated for too long so you may have to re-bond them after the castration and keep a close eye on them for a few weeks after they've been done in case anything starts and the stitches get pulled out. For any future vet trips they need for vaccines I'd advice you take both at the same time so they are always together as just taking one could mean a fight when you get back home and the one thats been on a trip will smell different from the vets hands.