I don't own a tandem, but I lived about an hour away from Acadia for 50 years. Leaving the area just a couple of years ago to move to FL. Still spend the summers near Bar Harbor. My wife and I have ridden the carriage paths and roads for the past 42 or so years. So here is a little advise from a Native Mainer. What months are you planning your trip? If any time during the summer months (mid June to mid Sept), personally I wouldn't ride the any of the park roads especially Ocean Drive or to the top of Cadillac. The roads while just repaved a few years ago are quite narrow without any shoulders. Ocean Drive is one way for quite a distance though. But there is parking on the extra lane. During the summer season the place is crawling with anti-bike drivers IMHO. Now the carriage paths are quite a bit different. But still crowded. It would be possible to use some of the more hard packed dirt trails near the visitor's center up to Witch Hole and down to Eagle Lake. Please be aware the a lot of the better runs are shared with horse traffic and their droppings. Just be sure to give them the right of way and announce your presence. Mostly the ones you will see are either rentals from Wildwood Stables or local owners that know the trails. However, poop is poop and riding though it isn't fun. There are a few places that are quite steep but worth the ride. I still have a few scars on my knees from my last fall and that was on a super Mtn bike due to loose rock/gravel and dodging horse waste. You would be quite brave using 28s as I wouldn't use anything but a Mtn bike on the regular paths. You might be better off renting a couple of MTN bikes in Bar Harbor itself. It's a beautiful area and the biking is great you just need to be aware that their is a LOT of vehicle traffic that is NOT real bike friendly. When in town don't forget to stop in at two cats for breakfast or lunch. Enjoy your trip. You can pm me if you want to know any more.