Creating a shell from 2 faces which were created with ThruSections and MakeFilling

I would like to create a shell from 2 faces.
One face was created with BRepOffsetAPI_ThruSections, the other one with BRepOffsetAPI_MakeFilling using an edge from the first face and an additional edge.

Unfortunately, sewing the 2 faces returns a compound, not a shell, even if I sew with high tolerance.
Does anybody have an idea why?

How can I get a shell from these faces?
If I simply create the shell manually with TopoDS_Builder and add the two faces, the shell is not valid because the faces are not connected. So, it seems that the result of BRepOffsetAPI_MakeFilling does not contain the given bounding edges but creates new edges.

I append a brep-file with the compound of the two faces.

Thanks,
Timo