Wrong?? If it works for how you use it and you're not worried about bumping into the frame while it's balanced on the open clamp's jaw then you're good to go. However, I'm not sure how well I'd be able to do any meaningful work on a tandem that was not firmly clamped by a workstand and/or also resting a wheel on something to give it added stability to compensate for the length of the frame.
All that said, any time that you have something "balanced" implies there's a level of risk acceptance going on. The only time I've ever dinged or nicked a tandem frame is when I relied on balance... so consider me a non-advocate of things like balance or friction standing between my tandems and falling against something that will leave me with a permanent reminder of a shortcut that bit me.
Just my .02.