5.1. A single crew operation aircraft can only have one crew member I believe?
I think you will find that the aircraft is certified for operation by a minimum of one crew. It is then up to the operator if they want to use more than the minimum; however the regulator may well say that any crew above the minimum required is not recognised as experience for revalidation purposes.
A FRTOL is a flight cre licence and a second person in a SPA could be regarded as a crew member if they were exercising the privileges of the licence; it would even count for revalidation of that licence!