Before trying a complicated example, I suggest you try a simple case generated within Caedium:

1. Create an outer box
2. Create a face fully within the box
3. Set the Topology Only property to on for the Boolean->Subtract tool on the Geometry Tool Palette
4. Subtract the face from the outer box - this will embed the face in the volume
5. Apply the Faces->2 Sided tool to the embedded face to convert it into a double sided face (membrane)

For more details on topology-based operations see "Topology Operations".

Then try simulating a flow through the box over the membrane in the usual manner similar to the tutorial "Incompressible Flow Through a Pipe into a Box".

Notes

• Importing 2 co-incident faces as a membrane is not the way to proceed. You need to explicitly convert a single face once it's embedded in a volume into a 2-sided face as outlined above.
• The embedded face has to be in free space - no other coincident or intersecting geometry.
• Order is important, first create the embedded face then make it 2-sided.
• Only a face embedded in a volume can be made into a 2-sided face.