We've got a Rhode gear limo for our tandem and fitted it as Michel suggested. The biggest issue we had, was that because of the geometry of the frame The Blackburn EX-1 rack needed to be modified slightly, so that the seat didn't slope forwards too much. It still slopes slightly, but the adjustability of the seats backrest means that our son is comfortable.
Oh, I've also found no problems with balance etc that lead a lot of people to suggest trailers as the best way to travel with small children. We do use lowrider panniers at the front whaich may be of some help here.