I just worry because a) the carrier is *just* big enough for both of 'em to lay down - no bigger, and b) they can't use the cat box whenever they want. And there's no guarantee they'd use it when I stopped, so...well, I worry. I can deal with cleaning up messes, but I don't want them miserable. On the drive out, we let them out after we got well underway each day. Probably against the rules, but it worked.
On the way out here, they slept most of the way. And probably I'm worrying needlessly about their comfort. After all, they spend most of their time when I'm not home curled up in a little ball together on the bed...don't use much more space there than they would in the carrier.
I'd say to leave them in the carrier and have it up front with you if the front passenger seat can accomodate it.
They'll feel safer seeing you there and you can talk to them and have a feel for if they ~really~ need out or are just being bratty. :-P