It shouldn’t be too hard or stressful at all. I take my rabbits on a 7.5 hour drive from NY to PA (to visit family on a long weekend, which means a return trip just a few days after we arrive) and they’re fine in their carrier. I try to offer lettuce, mostly for the water content, but they’ve never eaten it. Sometimes they’ll eat hay if I put some of that in the carrier. I don’t offer water because they don’t drink it (even at home they probably drink once or twice per day – a 7 hour car ride isn’t too long to go without water). You can try offering it during rest stops though.
Unless your rabbits are extremely nervous and very easily stressed, there should be no reason to need sedation or do anything special at a rest stop. One of my two is fairly nervous and anxious and she does fine on trips. When we stop, one of us tries to stay with the car while the other uses the restroom. We have stopped to get food and left the bunnies in the car alone as long as the weather is nice and windows are partially down. I think flying would be far more stressful for them and for you and it is difficult to find airlines that allow rabbits in the cabin.
Whether to put them in the same carrier or not is up to you. It would be a fantastic bonding experience and I would probably do it if you were just taking a trip, but since you’ll be moving and stressed out in general, it might be easier for you to just keep them separate – one less thing to worry about.
ETA: You don’t need a special carrier for them either – whatever normal sized cat carrier you’d use for a vet visit is fine. Some people worry about keeping them in a small space for too long, but bunnies feel comfortable in small enclosed spaces and it’s safer for them in case you have to slam on your brakes or something.