I hope this makes sense - in an orthogonal view of the model (top/front/right etc) does the split line appear as 1 zig-zag line or can you see the lines at the back.
If it's one set you can include the edges and create a splitting surface that projects through the part.
Alternaively can you offset the whole thing inside or outside. If that works you have 2 sets of edges that can be used to generate surfaces for stitching then splitting.
Another way is to use this model as a master then use part copy to extract the surfaces you require.
Hi Alan,
It looks from your picture that the part has thickness.
If this is the case you should be able to create a blue-surf that uses the outside and inside edges as the boundaries. You can then use that as the split surface.
Could you make a parasolid of the part that I can open in V20 and post it here? Parasolid version is 18.