Seat tube angle on almost all production tandems is 73 degrees front and back. I think large Burley's might be 72.5. Seat tube angle on tandems has almost no effect on the steering. The bike is too long and changes in how much weight is on the front wheel vs the back wheel will be negligable. Most folks adjust the seat fore-aft to the KOPS of their single bikes anyway.

ElRey is talking about single bikes.