About tandem designing; On the bottom bracket height; It depends on what type of tandeming that you want to do. A road tandem should have a bottom bracket height- ground to to crank center (I see that you are tall, so I am assuming that you may use long- 175 cranks) of 10.75" higher means that with your preferred seat height it makes it higher off from the ground and a higher center of gravity (not desirable). A mountain bike tandem should have a higher bb height. The head angle is pretty critical being off A few degrees is not good. On the top tube; It is critical to have the two top tubes intersect on other sides of the captains seat tube. If you put them at separate heights you loose significant stiffness via the seat tube twisting. (definitely not recommended) I have seen other people do it. Insert frown.