I know that there are a couple of us that have tandems- bikes only a few inches shorter than our smarts! A roof rack is the only option for us, unless our bike frames have S&S couplers to allow the frame to be disassembling into small pieces, but spending more time assembling/disassembling a tandem than riding isn't very worthwhile.
Another reason for my preference is keep access to the rear cargo area clear. I have had rear bike racks before and they could be a nuisance when getting to the rear hatch or trunk, but not impossible.
it sounds like the Q116s combined with a stretch kit is the way to go for tandems, since Yakima does not recommend carrying a tandem when the towers are closer than 24 inches. I first though I wouldn't be able to carry my tandem, now there is hope, thanks to Oliver!
Tom