In the civil world, the requirements are dictated by the certification of the helicopter - Part 27 and Part 29. They have requirements for single pilot IFR use in terms of stability and handling and redundancy of systems.
For example, for the Bell 430 to be single pilot IFR, it needed to have a second AFCS fitted - the two pilot version was OK with a single AFCS. The logic was that if the single AFCS failed in the two pilot version, the second pilot could handle the radios and housekeeping stuff while the first pilot kept the blue side up. If only one pilot was on board, the handling without AFCS wasn't up to the requirements, so a second AFCS was needed.
The military may have some different approach, depending on the country involved.