Has a fixed-wing aircraft ever been conceived, prototyped or built that features multiple turbines powering a single propeller? The concept of a twin-engine aircraft with a single propeller is intriguing to me. Picturing something like a Pilatus PC-12 with two engines under the hood and a single prop might have some advantages. I come from a fixed-wing background and know little about helicopters, but if something exists I suspect it is a rotor wing.
The LearAvia LearFan 2100 used two separate PT-6B to drive a single pusher propeller through a common gearbox.
LeraFan 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.
Well multi engine helicopters like the Bell 430 or the Westland EH101 do it all the time.
I don't recall any current production fixed wing aircraft using multiple engines to drive a single propeller, but the ill fated LearAvia LearFan used this propulsion arrangement, using two PT-6 engines to drive a gearbox connected to a pusher prop.
Yes, see Soloy's Dual-Pac concept.
Image courtesy of Soloy
The Soloy Pathfinder 21 is powered by the Dual Pac PT6D-114A. It is essentially an extended and redesigned Cessna 208 with exactly what you describe: two PT6 engines geared to one prop hub. I understand that the project has not progressed beyond the prototype built.
Image courtesy of Soloy
The Dual-Pac was also tested on a DeHaviland DHC3 Otter platform:
Image courtesy of Soloy
Quoting from the Dual-Pac page:
Soloy Dual Pac – Twin Engine reliability with single propeller symmetry. The Dual-Pac was designed, patented and Certified by Soloy to safely combine the output of two independently operating Pratt Whitney PT6D-114A engines for a single propeller output. The system was developed with extreme redundancy so that single engine operation is not only safe, but able to be done intentionally under certain conditions.
The Ayres LM200 Loadmaster was a similar design concept, though a clean sheet design using a different powerplant and also ultimately unsuccessful for economic reasons.
See also the rotorcraft applications of the P&WC PT6T Twin-Pac, such as those that Carlo Felicione mentions.
Well, there is the Armstrong Siddeley Double Mamba engine.
It powered the Fairey Gannet
It wasn't a true single prop, but rather a contra-rotating prop (which amaze me).
Found a few more, some being single contra-rotating, others......multi-multi-engine?
There is one example of the twin turbine to single shaft Allison T40 powering a single propeller aircraft...the Republic XF-84H
The thing was apparently as loud as the Tu-95, due to having a prop that spun at supersonic speeds on its outer edges. And the prop created a continuous visible shockwave.. Personally, I am not sure if you would classify this as an aircraft, or more of a shock/awe weapon.
A similar concept was adopted on a bomber used by the German Luftwaffe in WWII. The bomber, named Heinkel 177 "Greif" was a four engine but had just two propellers since a pair of engines was mechanically coupled in each of the two nacelles. However coupling mechanically two engines on a single propeller revealed as a source of troubles (e.g. overheating occurred frequently on the rear engines) and the design proved to be unsuccessful. This probably explains why it has not been used any more since then.
Kamov Ka-26 (1969, 816 built) is powered by two 325 hp (239 kW) Vedeneyev M-14V-26 radial piston engines mounted in off-board nacelles, connected by a transverse shaft which drives the co-axial rotors.
According to Wikipedia, the reciprocating engines, although more responsive than modern turboshafts, are relatively maintenance intensive. The Ka-26 is underpowered with its two radial engines, especially when used in cropdusting role, where excess payload is common. No other helicopter exists in the world that runs at constant 95% engine power for most of its flight regime. This leaves the pilot with little power reserve for emergencies. Due to frequent overloads, the interconnect shaft which joins the two engines is prone to breakage and requires frequent inspection.
NOTE An edit to the question makes this answer nonresponsive.
The Fairey Rotodyne probably counts, depending on your definition of "single rotor".
In level flight it was essentially an autogyro, driven by twin turboprops on short wings, deriving a fraction of its lift from these and the rest from the undriven rotor.
To achieve vertical takeoff and landing, the rotors were spun by tip jets, fed from bleed air from the turboprops, augmented by fuel burnt in the tip jets.
Although it seems to have been technically successful both in lifting capability and speed records for rotorcraft, the tip jets made it unacceptably noisy at takeoff and low altitude, precisely where noise was most objectionable.
I wonder if bleed air alone could work nowadays (given higher pressure ratio turbines) to reduce the tip jet noise.