Currently have the Rockymounts Tandem roof rack. I can easily mount our 45lb tandem solo on our tallish Subaru Forester. The trick for me is using cross bars that extend 6" beyond the edge of the roof. I think the tandem's front chainring hits the car window first if the rack is further inboard. I'd recommend the rack, but I'd also consider hitch-mount options.
We used the haul the tandem on a standard 2-bike hitch rack behind a Mini Cooper. The wheels stuck out a little on both sides. It was really quick the load and unload the bike, and never gave us any trouble.