I made the two signatures of five A5-sheets of 90 grams, folded. I used a piece of photo board that I had lying around for the cover. I scored three lines in the 'spine' of the cover and folded the middle line inward. Then I used a modified version of the pamphlet stitch to bind the two signatures into the two V-shaped folds of the spine -- binding the two parts of the spine together.
This is what it looks like from the top.
Close-up of the W-shaped spine.
When you open the booklet up, you see the two signatures separately. I think I can do something with that: it makes for a nice, natural separation. Perhaps the left signature has to be read from right to left, so that you always have to start in the middle?
The stitches. The thread runs through the signature, through the V-shaped part, into the other V-shaped part of the spine, and then into the other signature.
Close-up of the thread running through the spine.
What the final result looks like.
I like it: it binds the two signatures together but keeps them separate. That offers opportunities that can be used in a creative way.