Subtraction of surfaces

Hi

I have 2 surfaces in stl format and want to subtract the one from the other to create a single stl file. I tried the Booleanoperation.xml filter but I seem to not get the desired result. Is there something I am doing wrong?

I chose the first surface as the outer STL and the second surface as the inner STL and under the properties (Boolean operations), I chose difference.

Thanks