In my company we operate a CS-23 "bizjet" (it's a turboprop but the operation is the same as OP describes) under SP operations but there's always two-men crews in front. Technically the guy sitting on the right hand seat is a passenger I believe. Actually I'm not sure of his "legal status" in terms of insurance coverage etc.
But I think it's absurd that SP operation is even allowed given the amount of work involved. It's ok for a short ferry flight but for longer trips it makes no sense.