Face-Face Intersection

Hello
I´m trying to calculate the intersection between two faces. (here a very simple example http://rapidshare.com/files/248138779/Sample.jpg.html - I need to calculate the red face) The faces can be more complex but I didn´t get it work yet for the simple one.

What I tryed:
1) BRepAlgo_Section -> calculated the section and used it to build the face. -> doesn´t work because the resulting wire is not planar
2) BRepAlgo_Cut -> seems only to work good with Solids. Maybe I´m using it not correctly.

Can someone give me a hint????

Thanks in advance - Stefan