Hollow face

[micheus]

Select a set of edges and loop cut them.

But, if this skull was "solid" that would be better if you had used it before create the inside mesh.
In such situation, after make this big loop cut you could select the big face and other faces to be opened and use Intrude (to create the inside mesh) or set them as Hole.

That green area is a new face created by Wings3d. Remember, Wings3d works with manifold mesh, so you will never get an "open" object. You just get it using the Hole command.

In the current situation, if you want to keep the inner mesh - that means your mask will have a thickness - you must to loop cut the respective faces in the inner mesh. This way you will get two big faces opposed each othef; select both and is Bridge to create the "hole".

If you don't want the inner faces and intent to work with only the external faces and use then Hole command, then there are more work to do.