When finished, and the endpoints share a common location, the corner looks like the figure below.
Why is this preferable to assigning a single mullion to one curtain wall and leaving the adjacent curtain wall without a mullion? That would require that the edge handle of the CW without the mullion to be manually aligned to the edge of the mullion where it crosses the panel's path; a feature that should be automatic in Revit. Changes to the mullion profile won't necessarily be reflected in the open-ended CW and is an opening for errors.
via Jon McFarland at
CurtainwallBIM: Mullion Halves