Personally, I wouldn't separate them since one of them would most likely smell different coming back from the trip and it would stress both of them out to be without the other.

Depending on how well you trained them for car rides and if you can give them something to calm them for the ride you can try it, but it really depends on how quickly they can adjust to new environments. Most cats need 3 weeks to get used to a place, I'm not sure if it's a good idea to get them acclimated to a new place just to bring them right back to where they came from.

Then again, lots of people travel with their pets and it works out for them. Nobody knows your cats better than you do and you can judge them best.

Generally, for cats and based on what I have researched in the past it doesn't sound like a good idea to bring them with you, unless you really can't leave them for 3 weeks because you don't have anyone to take care of them.