Common mistake. Single pilot aircraft may be flown single pilot, but there is no regulation stating that they must be flown single pilot.
Many light twins and small jets have single pilot authorisation, but are flown as two pilot operations for safety/passenger insurance reasons.
If two correctly licenced pilots are assigned flying duties on a Cessna 150 (with only two seats) they are perfectly within their rights to log commander and co-pilot, if that is what they are actually doing.