Thule makes a reasonable roof mount rack. It requires attaching a nylon strap from the handlebars to the captain's seat post to lock the bars into a straight ahead position. Then you remove the front wheel. The rack has a hinged arm the swings out 90 degrees from the rack. You attach the front fork to this arm, with the rear wheel still on the ground. Once the fork is locked into place, you lift the rear portion of the bike while pivoting the bike over the roof. You set the boom tube into a padded cradle on the rack and lock it down with plastic straps. Then you lock into place the hinged arm that holds the fork and you are ready to go. It is much simpler than the above description sounds and easily managed by one person. Check the Thule website for illustrated details.
Note, if you are going to put your tandem on the roof, I strongly suggest a bra or similar covering for the front of the bike. While the rack is rock solid and never a source of worry, bug and pebbles hitting the bike did make many small paint chips before I installed a front cover.