Solidtrim on two Brep, leave the trimmed one open

Hi all, 

I'm trying to trim a brep (made from an extrude surface) with another brep (made from a polysurface) it works perfect with the Solidtrim tools in grasshopper but... meanwhile it does the cut it also close the brep.... OR i don't want to add extra surfaces from the cut....

Any advices ?

Thx in advance too all