Baby bunnies are usually ok to stay together until 10 weeks old, possibly a little longer. It all depends on the hormones coming in. When they start trying to hump each other, they definitely need to be separated then, as that can cause a fight to break out, which could result in serious injuries. Though you may be able to still allow them to have play time together if they can refrain from trying to just hump each other the whole time. Most hormonal male buns can't. I had some baby buns that I separated at around 11 weeks old, but still let them have time together for a few weeks after. Of course this needs to be
very closely supervised if you do try it.
Usually their testicles don't fully descend until around 4 months old, so most vets won't neuter before then. And some say 6 months old, especially if you have a smaller breed rabbit. You'll just need to ask the vet the age they'll neuter at, and I would recommend finding a very experienced rabbit vet for it. It minimizes the chances of things going wrong with the surgery.
https://wabbitwiki.com/wiki/Spaying_and_neutering_rabbits
http://www.bio.miami.edu/hare/opcare.html
Here's a list of known rabbit vets, though I would still recommend doing your own research to find the best one for your bun.
https://rabbit.org/veterinarians/
There's always a chance your rabbits will rebond after being neutered, but there's no way to know for sure. It all depends on their individual personalities post neuter. Fixed male/female tends to be the best match, but 2 fixed males can work as well. You just have to wait and see. You do have to wait a minimum of 4 weeks post neuter, for hormones to fade before attempting the bonding process, though sometimes waiting 8 weeks is necessary. Basically long enough that they're no longer just immediately wanting to hump each other incessantly the whole time. But even with fixed rabbits some dominance humping will occur, though it should be minimal.
https://rabbitsindoors.weebly.com/bonding-bunnies.html
http://www.cottontails-rescue.org.uk/information/bonding-bunnies/
https://wabbitwiki.com/wiki/Bonding_rabbits_together