It all comes down to HOW the helicopter is listed.
A helicopter-independent of itīs type or weight-can be listed as a "multi pilot helicopter" or "single pilot helicopter"-for all, or only for a few operations (e.g. "VFR: 1 pilot, IFR: 2 pilots).
This needs to be written either in the OM or the FM, and needs to be approved by the authorities.
Once this is done, there is no way an additional pilot can log P2 (in an approved SPH-unless under instruction, but then NOT in commercial air transport), or that a helicopter can be flown with one pilot (if approved as MPH).