This all depends on the Model, what options it was ordered with, the type of operation 91,135 etc. And client requirements as well, some contracts stipulate Multi-Crew.
Even the B1900 is a Single Pilot machine ( essentially a King Air on Steroids), I once met a Pilot from Raytheon who had just repositioned an aircraft to Brisbane, Australia from Witchita, Kansas Single Pilot. Obviuosly it was 91 Ops and he would of had an ATP.
A classic example of options ordered, was when the US ARMY ordered A90 King Airs in the 60's ,they were configured for 2 crew ops, ie dual instrumentation, sources etc and repositioned fuse and breaker panels.
But generally I think you would have to be rated on the Airplane before logging rightseat, depending on your country requirements.
