I don't know if you figured it out yet... But here it goes:

1. select edit point selection method on the first circle
2. select the 2 edit points on the x-axis
3. edit curves > detach curves
4. select the lower part of the circle and delete it
5. repeat step 1 to 3 for the second circle
6. select the upper part of the second circle and delete it
7. now select the first circle half
8. shift-select the second circle half
9. edit curves > attach curves

that should be it... you can now play around with the curve's shape by moving edit points or CV's