Actually, my second floor joists will be paralell to the walls that will support my rafters. The width is 16' and the depth is 20', so I am putting the joists across the 16' dimension. The rafters will be front to back, so they will span the 20' dimension. I will have to figure out how to attach the front and rear 12' high stud walls to the second floor decking at 8' above the first floor decking, so that the rafters will not cause the front and rear walls to spread. I guess that I could attach a ledger to those walls, and then use the floor decking to hold the walls vertical. Is that stong enough?
This method you suggested sounds like what I have read about as "balloon framing" versus "platform framing", which is what I am used to.