Elevators are not usually very tandem friendly. Santana and Co-Motion make tandems fitted with S&S couplers that break apart, but this is not the same as folding. The only folding tandems, to my knowledge, are the Bike Friday tandems. Those might be an option for you. Humping a full size tandem around a 780sq./ft. apartment doesn't sound practical, I think the basement storage is your second best bet, after a folding one. After some practice and experience a tandem captain can ride a tandem anywhere anyone else rides any other kind of bicycle. Good luck.