In my opinion, as long as the kitties see each other on a somewhat regular basis so they know each other, why not take them along. I believe that cats get used to things. For instance, long ago when Tubby was young, I lived in Green Bay, WI. I would often take Tubby when I went by my mom's house - which was at least once a month, and was a 30-40 minute ride. At first he didn't really enjoy the car ride, and once we got there he was a little tentative, but he soon got over that and I really believe he ended up looking forward to the visits.

So as long as the kitties seem to be enjoying it - why not?