No enough info to offer any solution to your dilemma, but someone recently shared a link to an MTB forum on the other side of the pond that offered some some interesting single bike home storage solutions on my off-road tandem forum that also have merit for tandems:
http://happymtb.org/forum/read.php/1/483001
As for your situation, a photo or some dimensions on your apartment as well as some info on your tandem (type, weight) might help. Where do you keep your other bikes, assuming there are some others?