One answer may be found in the ill-fated, eventually abandoned
LearFan. Brilliant concept - twin engines driving a single pusher prop - not unlike a multi-engine helo in this respect.
But the FAA COULD NOT BRING THEMSELVES TO CERTIFY IT as a twin because of the common gearbox and prop. Thus it died on the vine - and probably brought about its inventor's death as well.
Hidebound, unimaginative, CYA regulators and regulations have and always will determine aviation's future.