The pilot who is qualified to fly the C152 would have an SEP rating and therefore qualified to fly a PA28. (Whether that pilot is permitted to fly the PA28 is an entirely different matter and down to the owner or operator of the PA28. It has nothing to do with Flight Crew Licensing.)
The pilot who is qualified to fly the PA28 would have an SEP rating and therefore qualified to fly a C152. (Whether that pilot is permitted to fly the C152 is an entirely different matter and down to the owner or operator of the C152. It has nothing to do with Flight Crew Licensing.)
Both the C152 and PA28 are Single Pilot Aircraft (ie. they require only one qualified pilot)
There can be only one PIC at any one time. Who is PIC when is determined before the flight takes place.
The 'other pilot' is not a member of the Flight Crew because it is an SPA.
The 'other pilot' is merely a passenger.
Now if either one of you is an FI or an FE then that would be a different matter...
Cue 'the rule-benders'...