Rhinoceros

I quote this from Off BroadWay

1. Add one curve vertically between the lower branches (white).
2. Loft both branches together with this curve (red surface)
3. Trim this with a plane.
4. Loft the upper curves.
5. MatchSrf for tangency the upper to the trimmed loft.

I added a knot horizontally in the lower span of the blue surface before matching to limit the area distorted by Matchsrf.