Doesn't Alpha Flying out of Manchester, NH fly the Pilatus PC-12 single-engine turbine under part 91 with 2 pilots? Their ops specs say that the 2nd pilot is required for their operation while the aircraft is certified for single pilot operations. The 2nd pilot legally logs SIC time because of the ops specs. So in this case, even if the aircraft is not certified as requiring the SIC, SIC time can be legally logged.