In a nutshell - does your operations manuals require two crew when carrying passengers? Speak to your chief pilot for guidance.
If YES - the time counts both for P2 and multi-crew time [500 hours MP required for ATPL issue.]
If NO - unable to log time as P2.
Simple - black and white.
This "hybrid rule" applies to ac under 5.7MT, and affects types like the Otter operating on a JAR AOC.