They're harder to string than they are to build in my opinion
Just make two D bows. The main bow should be a R/D bow, tillered out to full draw with a nice graceful curve and about 10-15# underweight. The back bow should be shorter and highly reflexed and HEAVY in draw. Just tie the handles together and make some cords to connect the tips (the lower the stretch the better). Tweak as necessary and balance them to full draw.
My theory is that if you can get a back bow that stores enough energy and is elastic enough, the main bow will draw like a heavier bow, store the energy of a heavier bow, but have the lower limb mass of a lighter bow. Of course... that's alot easier said than done