You can't really create a NURBS surface out of just two curves. I suggest attaching the two surfaces you have using the Edit NURBS>Attach Surfaces command (I think that's it, anyway). First select the adjoining isoparms on the two surfaces, and then run the command. You may need to reverse the direction of one of the surfaces (I doubt you'll really need to though). This will give you a nice smooth transition. If you hit the option box on the command you can choose whether or not to keep the original surfaces, as well as some other options I believe. Arguably the best way to do what you are doing, though, is to use surface trims to remove the indentation at the top, rather than trying to have one contiguous curve around the left, upper, and right edges of the surface. Create the left, right, upper, and lower boundaries, then do one of the several operations for creating a surface from four curves (birail, square, etc.). Then project the indentation curve onto the resulting surface and do a trim to get rid of the unneeded area. Hope this helps.