You can keep them together for another 1-2 weeks or so, then you need to seperate them or you will be wading in baby bunnies up to your knees before they are 4 months old.
I would keep them with each other in sight, but do not underestimate their ability to climb and jump. If he get's neutered first he'll have to be kept seperate for another 3-4 weeks to become infertile for good. The doe might need some days alone after her spay to heal up properly.
Over here, some vets do neuter bucks early, before puberty hits, starting at 8 weeks. This has several advantages, like never learning hormonal buck behaviour, no seperation necessary, no risk of surprise litters, etc. But for whatever reason that doesn't seem to be popular in the US, as far as I know?