I build my own roof racks with a fork mount in front attached to front cross piece of standard minivan and tire channel anchored to rear cross piece. For a tandem I had to turn it around and put the fork in the rear because the rear tire was extending back to far --preventing the rear hatch for fully rising.
(I get six people and full luggage inside a minivan with space for 7 bikes outside (4 on back, 3 on top).