Most people don't know this but rabbits are social and DO like being in pairs, however...
If you keep two of the same gender together, when they hit a certain age they will start to fight. This could be 3 months or it could be 8, depends on the rabbits. Usually it's 6.
Rabbits of opposite genders will breed (duh) and if kept together 24/7 the buck will over-breed the female, breeding her over and over again as soon as she gives birth, unless you have a HUGE area for them to move around in. Plus you will need an outlet for the kits whether through pets (which sometimes also means they end up at animals shelters) or meat (but Giant rabbits are a bit bone-heavy to be good meaties, but it can be done).
The solution to both of these problems is to get them fixed of course, but most vets won't fix rabbits.
Rabbits will also be social with their people and other pets and this can be an OK substitute.
Good luck!