The trick is to click "Face(s)" and not "solid(s)"! (else function don't work)
here "Inward" + "Grooved" on the top face
Union Top and Bottom solids
then Shell the top Face + Fillets on some edges
(with the real time free Simlab Composer lite max 1920 * 1080)
If you want to get some control over cut line position on the helmet, you can project a curve to the side. Since it will be a closed curve, split it in 4 pieces with trim points, so you get 4 sides for network (I highlighted front and back pieces). Add profiles roughly in the middle, they will define depth of the cut, my example is much deeper than on your model.
Narrow cut ("mouth") is obtained again from side projected curve, used as a rail for the sweep.
helmat01.jpg