@epidiah Oh, I think I misunderstood the assignment. So you have two lines (like stroke objects with a width and colour and all that) and you want to bring down a cookie-cutter and chop a chunk out of them?
This seems hacky, but what about if your "mask" shape was actually a rectangle that filled the canvas, and had a hole out of the middle of it. That way the circle in the middle is actually a negative space so the clipPath shows the "outside" of circle?