Pending EASA rules:
The requirements for two pilots in the EASA Operational Rules are (apparently) substantially the same as the JAR rule (contained in JAR-OPS 3.940(b)(2) and (3)) - i.e a configuration of more than 19 pax or, for IFR, a configuration of more than 9 pax.
A pilot flying SPIFR requires additional training elements and there are minimum qualifications. The helicopter would have to be certificated for flight in IMC and meet the additional requirements for SPIFR.
There is no requirements for two pilots other than that stated above or when required by certification - i.e. there is no requirement related to mass.
Jim