First, two NURBS surfaces (deformed grids)
With the Ruled surface tool, we select the left edge of the right surface. As you can see, it select all the edge...
Clic on the right edge of the left surface. In some case (mine...) the ruled surface may be crossed. It's because my two surfaces are inverted (My second surface is a copy of the first one, but with a 180° rotation).
Now, we have to correct this "error": click on the arrow parralel to the edge: the surface is now uncross
As you cansee, the result is not really nice: on the connection of the surfaces, the angle is really sharp. To smooth the connection, you can clic once on the perpendicular arrows and Amapi will modify your surface to be smooth (tangent to the connected surface).
Do the same on the other edge.
Now, weld the three surfaces. what a very nice result!