How about this for thought:

I roughly set up 4 circles - (yours could be Offset Curves created off each end Isoparm of each object), and then created a bunch of reference curves between the circles. All the reference curves are snapped to each circle to ensure a nice smooth edge later.

I then lofted all the curves except each corner area (that I have marked in red) to get a surface that follows the curves.

The corner area was lofted separately because including it in the original loft caused the corners to be rounded.

With the surfaces lofted I would then do an Edge Stitch to get everything looking nice and would then select the edge Isoparms of the lofted surfaces and do an Attach without moving.

I haven't finished it but might carry on in my spare time to see what happens.

This should give you a centre surface between your already existing ones which you may then be able to use to attach everything together.

Below is an example of what I did VERY ROUGHLY, without the stitching and attaching.

Hope this helps.

Steve