My 2 cents: when you rent/borrow a tandem, try to get one that isn't a total clunker. We rented a clunker and just barely decided to go ahead and buy one -- I was concered by how terribly it handled (it was exhausting to steer). By contrast, our CoMo Mocha rides beautifully.

(You may not have an option, but we did and I've realized I was wrong to rent such a cheapo.)