Sewing two bulky seams together, generally means that one seam will be out of alignment. I have clients who need to have extensions added to most dresses because of their height. Once the extensions are added to the front and back panels, the side seams need to be restitched. Generally with this much bulk, it will mean that the front or back panel seam will "move" and when turned to the right side the seams are out of alignment.
This video shows how to line up the seams using only a pin, and how to ensure that they are always even.