In my experience, small bunnies need every bit as much space as a bigger bunny, they tend to be much more active and love to run. They may not need as much headroom but definitely need just as much overall space.
Are your bunnies neutered? You may be able to ask a rescue to try rebonding them for you in return for a donation, so that they can live together. There is a bit of a 'technique' to it and it sounds as if it hasn't been working for you at home, so it might be worth trying a rescue before giving up and separating them for good. Bunnies are social animals and if bonded confidently will usually far prefer living with another bun.