We rode our Cannonndale Road tandem on the carriage trails in August a few years back without issues. We ordered some Conti cyclocross tires that were marked as 42c. Unfortunately after they arrived (and too late to make a change) I saw that they were really 35c tires, marked as such molded into the casing, with 42c printed on the sidewall in yellow ink -- I think the 42c is the dimension including the knobbies. That said, they worked fine. We swapped out our regular 28c Gatorskins and installed the cyclocross tires before riding, so I can't tell you how the Gatorskins would have worked out, but from the look of it I was glad we were on bigger tires.

We didn't attempt to ride on the paved car roads at all, we started on the carriage trails from the visitor's center parking lot. I didn't think the carriage trails were crowded, we thought they were great. The regular roads didn't look particularly bike friendly, but we didn't give it close consideration because we were pulling a kid trailer behind the tandem, so we would have needed a LOT of room before thinking about riding there.

I think the tires we got were steel bead, which was cheaper and no problem since we had a car to carry them in. But plenty of cyclocross tires are available in folding models, so you could carry them and swap them if you wanted. To go bigger than that, I measured our frame, and I think it would have taken at least a 48c tire, but I couldn't find any of those that I could get quickly enough -- we had less than a week for prep. With more time, you could probably come up with bigger options. The stoker didn't complain about the ride of the 35c's though.

One thing we weren't used to as road riders was the dust from the carriage trails on the drivetrain. Folks with MTB experience will probably be more used to this and have a chain lube solution already in hand for those conditions.

We did LOVE riding on the carriage roads and want to go back and do it again. We just haven't found the right opening in the schedule yet.