Knit surface to form solid - error when a common s

Error: The pieces cannot be joined into manifold bodies because more than two sheet bodies vcannot be joined together at a single edge junction.

I am trying to form a solid body using 2 main surfaces with a partition between them and I am getting the above error. However I am able to form a solid with just the front surface/back surface and middle surface and all corresponding loft surfaces.

So my question, is there a way to knit the surfaces shown in the part to be knitted to try and form a solid? I need the partition to still be there because the front part will have different properties than the back part. Is this possible in SW? I can think of another option which is to duplicate the intermediate surface and 2 separate solids, but I would like to see if any one can help me with creating just one solid part with a partition rather than 2 separate solid parts.

The part is available here.

Please help!