I did a hickory bow that had siyahs. I flattened the last 3" of the back that was overlapped by flat underside of siyah which had a little notch drop down of about 1/8" to make a butt joint against the end of belly. Wrapped it all with sinew after glued together and no problem.
Imagine 2 capital L's spooning each other for the surfaces of the back, siyah, and butt joint, and then wrapping them up with sinew. Sorry, not good at describing this