Has a fixed-wing aircraft ever been built that featured multiple turbines powering a single propeller?
The LearAvia LearFan 2100 used two separate PT-6B to drive a single pusher propeller through a common gearbox.


LearFan 2100 in flight (picture source)

If you want to know why its official first flight date is December 32, 1980, read here.


LearFan 2100 engine arrangement (picture source)

This engine arrangement was the LearFan's eventual undoing when the FAA denied it a proper certification because of concerns that the single gearbox constituted a single point of failure.