It would appear to be FAA regulation that there always be two people on the flight deck. The language is slightly ambiguous, but seems to say as much:
http://fsims.faa.gov/WDocs/8400.10%2...16-Sec%203.htm
2287. PROCEDURES FOR OPENING, CLOSING, AND LOCKING OF FLIGHTCREW COMPARTMENT DOORS. ...
D. Certificate holders must have approved procedures for opening, closing, and locking flightcrew compartment doors. These procedures should be included in the operators operations and flight attendant manuals, and must include at least the following:
...
(g) Procedures for two-person flightcrews, when one flight crewmember leaves the flight deck (i.e., a flight attendant or other designated person must lock the door and remain on the flight deck).