Engineoff the concept of co-pilot time is attractive to many but be warned the FAA procedure of 'split' claim for PIC with two qualified pilots onboard will not count under JAR.
The closest equivilent is the Student Pilot in Command (SPIC) which has appeared with the advent of JAR regs, most agree this is of little value.
As to the 'complex' types defined as requiring a two crew minima under JAR speak to an expert who is conversant with JAR FCL 2 (Heli regs) as these are defined within. In general, the larger (7 pax +) types used almost exclusively for offshore support require a minimum crew of two and as noted by 212man OPC also required for any 'co-pilot'.
Just curious but your desire for multiengine time is bacause? If it is to assist your passage under a JAR ATPL(H) remember the JAR minima also requires defined IR time in addition to the defined multi crew/multi engine.
A complex subject as all have implied!
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Proper Prior Planning Prevents P*** Poor Performance........Do it right, seek advice!