The OGP study shows that the multis operated with two-pilot crews have a significant safety advantage over all other equipment. Off the top of my head, that's the only source of stats that comes to mind. There are apples and oranges issues in the numbers, for instance- flight profiles, ie- my impression is that all the two-pilot crews in the study are IFR crews, whereas you can't say that of all the twins. So, perhaps it's the IFR capability that's significant? That makes sense to me, too. I think some reasonable assumption follow:
IFR crews are generally better trained and prepared at each dispatch;
IFR flights face serious magnifiers of hazard;
And two pilot crews address the greatest hazard to any fligth- pilot failure of all sorts.
Pilot error, or just poor pilot performance, is the greatest threat to safety.
I can count on my fingers the number of inflight power failures I've had since '68. Systems failures, on the other hand, would be several dozens of occurences, and pilot mistakes of all types, thousands in my career. Look around, and I'll guess you'll see the same pattern. Twins address the least likely hazard, multiply the second, and geometrically amplify the third in my list.
What does work, apparently and in spite of the frequent clashes of personalities in the cockpit, is having two pilots addressing the challenges of safe flight.