Originally Posted by ROJA
I know that, but I've used it for singles with no problems and I've heard of others doing it as well. "
Well, then I guess you have your answer...
Seriously, it's a roof mounted rack that you can strap a bike to. Given that a piece of lumber bolted to a roof rack with a scew on fork mount and a toe strap for the rear wheel will hold a bike, it's a pretty safe bet that you can also strap one to the powdercoated, box section Sidewider. While I suspect that a few roof mounted rack systems have "failed" either because they were poorly maintained (rusted or damaged skewers/hardware/straps) or were poorly thought-out or poorly fabricated home-builts, the vast majority of bikes damaged in association with roof-mounted systems are the result of human error.
Back to your Sidewinder, yeah, I've seen them used for single bikes too and they sit very high up assuming the rear wheel is sitting on the boom support. There's also not a lot of lateral support for the rear wheel but it's a 20lb bike... so how much lateral support do you need if the fork's secured? Back to the height issue, you'll definitely have to bear in mind your overhead clearance is as it will be even more restrictive than when your tandem is installed on the mount.
Bottom Line: Aside from the Yakima name, there is nothing appealing to me about the design of the Sidewinder. Yakima obviously wanted to differentiate itself from the Atoc/Thule system and to steer clear of patent infringement issues and they have succeeded. Me, I was always able to use multiple single bike mounts (which is also something Yakima warned against as those had not be tested either) for my roof mounted tandem portage (up to 3 on the roof of our '97 Suburban at prolonged highway speeds and in adverse weather) instead of shelling out for a tandem mount. If I was compelled to shell out for a tandem mount I'd likely go with a non-swivel Atoc model which has a low profile, can carry short or long wheelbase bikes, and is aesthetically neutral for about the same money as the Sidewider. If a swivel mount was essential, I'd still go with the Atoc offering for all of the forementioned reasons.
More than you wanted to know...