We accomplish this by creating the profile for the male mullion half and assigning it to one wall and assign the female half to the adjacent wall. The end points of the walls need to be separated when the mullions are assigned but are moved together after. The figure below shows half of one mullion at an outside corner.
The figure below shows the adjacent curtain wall with the other mullion half assigned as the mullion profile. Remember, the endpoints should no be co-located when assigning the millions.
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. The figure below shows a similar scene with a single, full mullion at the edge of one of the curtain walls.