adot45,
I would use a square to mark the bottom shelf dado's left side on the top, again use the square to mark the top dado's right side on the bottom, measure the width of these new in-line dados, of course cut them both wider, then slip in the new divider.
Another easier approach: make a new divider that is wide enough to cover both existing dados, hold it up to it's final position, and mark the top and bottom to make off-set tenons, cut the new tenons and slip it in.

Good luck, be sure to post you final solution.