So the old tandem frame is just too short. The front rider has to be waaaay too far forward to keep the rear rider’s knees from whapping the front seat. I knew that it might be the case, but it was faster just to try it out than to do a bunch of measurement. The downside is that I’ve decided to put off any more work until spring. It would be major surgery to alter the frame and ultimately not worth it. I could build a properly-sized frame with new metal in less time than fixing the old one. I did get the seats completed, and I’m very happy with the results (I’ll post photos), so I think my time hasn’t been wasted.