I have a couple of bars in the garage (I inherited them - I don't know what the previous owner used them for, though I am still tickled by Vince's suggestion that it was for drying sails), and I can hang the tandem from them using straps so that I can adjust the height. The bars are high enough not to be in the way when not in use.
it's a handy, low-tech solution, if you have a garage and some bars.
The drawback is that it is difficult to excert much pressure on the bike in any direction, as it'll just swing away from you, but it's easy enough to drop it (or one end of it) to the floor if need be.