Hi,
I guess the best thing to do is build/buy the shed/playhouse now, house them together and then if and when they start fighting then seperate it with run panels or similar. However, if they're outside it's difficult to monitor whether they do start fighting and it could be several hours before you notice, in which time they could inflict serious damage on one another.
Otherwise, try finding second hand hutches/cages on eBay, gumtree etc. They can be cleaned and scrubbed with pet friendly disinfectant to remove any nasties lingering, but make sure you ask what the previous pet died from and avoid anything that could be passed on. One or both of these can then be used in the playhouse/shed as an 'indoor' house inside their house
And if you have a run attached (which you should!) then another hutch can be placed inside this to give them somewhere else to hide.
It's also worth asking if any vets will neuter&spay them earlier than the recommended time frame, just say you're worried about fighting and mating. Some vets do.
Also, just wondering why these bunnies? There are plenty of already neutered and bonded pairs in rescue centres all over I know you said you're rehoming them, but just thought I'd mention it. The buns might not get on, and bonding buns is hard for anyone!