Just to add a bit about frame materials to my postings....

Our steel tandems were great, no issues at all.

When we were looking for a go-fast, light tandem, we test rode a Calfee carbon that we thought we would love (scared we would love was more like it, because of the high dollars!), but we hated it. It was very squirrely and noodly, just not stiff enough at all, and we were a light team (DH around 160 and I around 115 at that time, we're both lighter now!) We test rode some steel Santanas, and they were fine, but when we test rode the Team AL (aluminum Santana, and the lightest bike they made at the time), we LOVED the stiffness. I don't like aluminum on a single bike, but on a tandem, it felt very reassuring not to feel any flex at all -- all the power we delivered to the pedals was transferred to the bike. I had a suspension seatpost, so any discomfort was eliminated that way.

So, materials you love in a single (like, say, carbon), may not be as desirable on a tandem, and vice versa. The longer the bike, the stiffer it needs to be, in my opinion.

Emily