Yes, joining the two meshes certainly did make it more complicated for me to convert them into NURBS surfaces, requiring some forensic analysis to extract the two base surfaces. If this is going to be a regular work flow, I suggest again that you convert the mesh results from Kangaroo to NURBS surfaces before making the two-sided “solid” that you are waffling.
A “Closed Brep” is a beautiful thing. 
P.S. To put this another way @ralscham, the code from the other thread that converts one mesh to a NURBS surface was quite simple:
Below is the code (surface_from_points_Ryan_2019Nov24b.gh) I posted in this thread two days ago, adapted from that simple model. The white group converts the bottom of your joined mesh to a NURBS surface, the cyan group converts the top surface of your joined mesh to a NURBS surface, and the yellow group is all the extra stuff I had to write to extract the top and bottom from the joined mesh and then loft a vertical perimeter surface to join the two NURBS surfaces into a “Closed Brep”.
Most of the yellow group wouldn’t be necessary if the two meshes from Kangaroo were handled separately, before they were joined together into a mesh that doesn’t contour well.