Joining 2 panels for a reversible ruana

I am familiar with sewing techniques & various seams, I have completed 4 panels for 2 ruanas to be stitched down the center of each back. I would like the seams to be clean and hopefully reversible. A french seam will be too thick (merino colrain lace) and I considered a decorative join with the selvedges meeting and no allowance, also the possibility of adding a decorative trim over the join. Any suggestions would be appreciated and for those ruanas using a lighter yarn how do you personally prefer to join your two panels in the back (please only discussion of seams and joins not how one could weave the back as one piece and then weave the front as two separate pieces) Thanks Dawn