back-to-back

1.  With backs close together or touching, as in In the first and second rows of the bus, the seats were back to back, an unusual arrangement. This term also can be applied to persons who stand facing in opposite directions and with their backs touching. [Mid-1800s]
2.  Consecutively, one after another, as in I'm exhausted; I had three meetings back to back. [Mid-1900s]

The adjective has one meaning:

Meaning #1: one after the other
  Synonym: consecutive