Spend your time looking for builders with a good reputation, talk with them about what you're looking for and then talk to a couple of owners to see if the builder met their expectations. That's far more important than the weight of a frame, given most of the high-end tandem frames all fall in the 5.5 - 8 lb range, regardless of material, e.g., Carbon, titanium, Magnesium, aluminum and even a few steel frames. Put another way, it's the difference between a 26 lb tandem and a 28.5 lb tandem where four water bottles add nearly 6 lbs to the total bike weight.