Hey, we picked up our kitty Ares from a breeder who lived 1000 miles away. (He was 3 months old at the time, and I think be younger can make travel easier.) As it turned out, he is an awesome and well-adjusted cat, which definetely helped, but here are a few things we did:
He wasn't freaked out by the carrier, since we sent it to the breeder months before, so that the carrier sat out was played in by Ares and his littermates. (If your kitties are not used to or not crazy about their carriers, I would recommend spraying them with Feliway, and leaving them out in a play area. Place a few favorite toys/treats in there to entice them into the carriers and to create positive associations.) You might also want to try a few short car rides with them in the carriers to get them used to it. Be sure to line the carrier with pee pads and soft blankets (and have extras in the trunk). You should also decide if you want a large carrier to place both kitties in, or 2 smaller carriers.
We also placed a disposable litter box with litter into the trunk, along with a scoop (and PLENTY of plastic bags). We stopped about every 2 hours to give him food, water, and a litterbox break. (When doing this, make sure all of the car doors are closed and the windows are shut.) We ensured he had a toy or 2 in the carrier, and rotated them out with different toys for interest. Not knowing how he would react to the trip, we purchased Rescue Remedy (which is supposed to calm cats) which can be rubbed on the ear, but he was super relaxed, so it was unecessary.
With such a long trip, you'll need to carefully plan your trip to include pet-friendly hotels. I would recommend booking in advance, as finding a hotel that takes pets can be difficult.
When you arrive at the hotel, I would take in their litter box, toys, etc up to the room, and spray some Feliway in prominent areas of the room. Then bring them up, and once you let them out of the carrier, be present to see how they react to the new surroundings. Once everyone is inside, make sure you put the "Do Not Disturb" sign on the door, so that the hotel staff will knock before entering the room. If you go out, I would put kitties back in the carrier. Hope this helps!
Good Luck! Sending you calming vibes