if duplicating were actually a good idea then surely duplicating the gearbox would make sense ?
each engine could have it's drive split and drive into each gearbox
each gearbox could independently drive the rotor mast
there would of course be a ton of freewheel units and run-dry oil pumps so every eventuality would be covered
no dafter than carrying 2 engines
the failure rate would be 1x10^-64 so at least in theory nothing can go wrong