Could one course round to stb, and the other round to port?

I'm thinking that the real hazard is downwind/reaching legs with boats of different performance characteristics (non-spin/spin). Separating this portion might reduce chances of collisions because upwind the performance differences are less severe and visibility is better.

with B marks on different sides of the course, you could have them going on different sides, and converge only for C-A upwind and start/finish?

Think of two ABC courses (one inside the other like curent layout) with the B marks in different directions. Spin boat course would have a very short B mark (just to move the rhumbline away from the start/finish bouy) to the left (rounding to port), and non-spin boats would have longer B mark off to right (rounding to stb.).

Spin boats start upwind first, which gives non-spin boats time to leave A to stb. and reach to B before spin boats start upwind again...