If the quantum gravity is mediated by the hypothetical massless spin 2 graviton, it would in theory behave as other particles. If there are two paths available for a given particle, there will be a superposition of the wave function of each path.
In practice, ain’t gonna happen. The energy of a single graviton is so small, it’s wavelength will be enormous. Not only would you not be able to create the double slit experiment, you wouldn’t be able to generate a single graviton, and you wouldn’t be able to detect it.
There is also a reasonable chance that a quantum gravity theory won’t conform to a particle view. Spacetime as we know and love it may be quantized with no exchange particle.