How long did you vet say to let him rest after being neutered? I thought it was a month, but I could be wrong as I’m looking at getting my girl done and that’s a more invasive surgery. I definitely wouldn’t put them together until he’s completely healed.
I don’t have a lot of experience getting rabbits to use littler boxes together. I find mine naturally started using the “communal” one, but my little buns still have one in their cage and my girl still has her own in her space as well. If you’re taking him into a new space, bring his litter box so that the smell will tell him where he’s supposed to go. That said, bunnies tend to decide on their own where they want to relieve themselves and you often have to accommodate or constantly clean up accidents.
Try two at first and hopefully you’ll be able to get it down to one if they both seem to prefer one over the other.
Hope that helps!