The ALEA requires one to be a Member to examine their secret stuff.....but I did find this study done by Bell Helicopters.
Measuring Safety in Single- and Twin-engine Helicopters.pdf - Helicopters
The basic question I think shall be whether Engine Failures in Singles combined with all other causes.....is greater than Twins despite Engine Failure related Crashes being less frequent but other Non-Engine Failure related causes generate a higher number of accidents.
Part of that might be the likelihood of IFR Operations by the Twins that the Singles do not experience.