In a single sliding application, one door leaf slides to the side of the opening. Therefore, two door leaves get splined (joined) together to slide as a single unit.
A pair of doors sliding open from the center is considered a Bi-Parting door. Doors slide on the same track and contact each other in the middle when closed.
Doors that slide in different planes (separate tracks) “bypass” each other. The track may be mounted on the face of the building, or within the confines of the jamb.