have you been to http://www.srv.net/~vail/ and looked at how he layed out his bellows? How I made my bellows was to glue the stiffeners to the outer layer, when it came to the panel where the seam is I only glued half the stiffener down thenI glued the inner layer on top starting fromthe same side that had only half the stiffener glued leaving the opposing half of the panel unglued. Then when I formed the bellows into shape I glued the unstuck half of stiffeners to the inner layer(after carefully marking where each point must reach) then finally glued the outer layer. I used a half inch seam for the inner liner and then an offset seam that ran 1"from the center seam at the front to 2" at the rear.
I think commercial bellows makers wrap the inner layer around a form glue on the stiffeners and the glue on the outer layer, this way you could have seams on opposing panels . Hope this helps