Here is the Tutorial.
Start SolidWorks in Part Mode.
Front Plane>>sketch. Make a spline.
Draw another spline tangent to this one.
Under sketch tab click the spline tool arrow and select fit spline.
Select the spline one by one. Check the delete geometry and uncheck the closed geometry. Keep the tolerance to a minimum.
Click OK and we can notice that two splines are merged into one.
Extrude the surface. We can notice that there is only one face.
If we hadn't merged the splines than we have two faces.