Good question. I had no idea this is a thing.
I will look into it.
EDIT: Appearently multiplayer also requires a different window handling for vendors. So I wouldnt make a big deal out of it. Since left and right window designs are similar, there is basically no change required, other than switching sides. Those windows just cant be open at the same time while playing in couch-coop.
You mean the second image, right?
If anyone is interested discussing the stash mockup: