I don't know anything about helicopter ops, but hope the following fixed-wing experience may(?) be of some help.
Before getting my ATPL, I built twin hours by flying with in a corporate KingAir. They were single-crew ops and also Public Transport so I couldn't fly/log on the PT sectors - although the experience was useful.
However, one sector was often empty and, with the company's agreement, it was flown as a private flight which I could fly with my then PPL Twin/IMC/Night ratings.
I obtained the CAA's agreement that I could log that time as P1 provided that no-one else was logging the same hours.
It obviously needed the cooperation of the "real" Captain giving up those hours but, as she was about to become my wife, that helped. (Some of the older pilots were also happy to help because they didn't need the hours.)
This was quite a few years ago - I don't know if it would still work. If it does, you need to find a friendly/older pilot who doesn't need to build hours, and a company who will put you on their insurance policy.
Before you spend money on a Twin Rating, check with the CAA that it's still permissible. The regs may have changed.