First, create two polygonal surfaces (deformed grids)
With the Extract line tool, Extract the border of the right surface.
Do it again on the left surface.
Now, we have two curves (in fact, it's two polylines) which are on the same place as the border of the two surfaces.
Hide the two surfaces before creating the surfaces between the two surfaces to avoid errors.
With the Ruled surface tool, click on the polylines, to create a surface.
Unhide the two previous surfaces.
With the Tesslate tool, option Tesslation by slice, Add 3 slices (the first in the middle with Shift key pressed, then, another two slices in the middle of the cut surface)
Now, use the stretch tool to move some vertex on the right slice to smooth a little the ruled surface created before.
The same on the left slice.
For the last step, weld the three surfaces and apply a smooth. That's it !
