If they are young and un-neutered, it's unlikely they are properly bonded anyway, so I would just take the one. You will probably need to keep them separate on his return anyway so that his surgery site can heal properly. For bonded adult rabbits, separating them for a trip to the vet can be a bad idea, because the rabbit who goes to the vet has been taken into a stressful situation without his/her bond mate who would help them to stay calm, and they may also come back smelling a little different which can throw off the rabbit who stayed behind. So when they're adults and properly bonded, I would be taking both. If they're only a few months old, you're probably fine to take just the one since as they mature an intact male is unlikely to stay bonded to a neutered one.