Maybe what i have been trying out, would help you. We generally need the two Glass Panels touching edge to edge, and the mullion is normally behind this joint. For this, if the Mullion Profile is about 50mm in width, we keep the Curtain Panel, 25 mm beyond the reference planes in the Curtain Panel Family. This way, since the mullion cuts the curtain panel, now it exactly seems joined to each other.
Look at the image attached below.