You can log time if the operation requires two pilots. The Beech 1900 is also technically a single pilot aircraft as well, but FAR 121 requires two pilots, thus the first officer can legally log all the time. Airlines that operate Beech 1900s must issue type ratings with the restriction "BE-1900 Second in Command Required" because of the aircraft being certified as a single pilot aircraft, unless the type rating ride is done single pilot.