From my perspective, riding a tandem is always better than riding separately on singles, regardless of the distance. Yes, at least one of you will probably go slower up hills than on a single. So what? You’ll be going as fast or faster on the flats and descents, and doing it all together as a team, sharing the sights, sounds and accomplishments instead of being separated by the crowds or hills. If you find tandeming better than riding separately on short rides, the same will be true on a long ride, maybe more so.