If it works, it works.

I've used all kinds of hybrid roof mounts to carry tandems. Our triplet is basically secured using a single bike fork mount and a pair of suction cup mounts. It's not pretty, but it works and it only cost me $120 given I already owned all the other bits.

Just be sure that you've done your homework on the loads and keep an eye on your "farmboy-engineered" solution, as a $6000 tandem will be a pile of $20 junk if it comes off the roof at 60 mph, never mind any damage it does to your vehicle or anyone following behind.

Theres a reason that nice folks like Charlie Buchalter can make a living selling tandem racks...