In popularized QM material I've read, it is a single photon interfering with itself, not necessarily with other photons. So for instance an experiment where only a single photon is fired at a time into the double slit apparatus will yield, over time, a visible pattern of interference on the recording screen.
But the thing is this : it is not really a single photon, or groups of photons. It is a single photon or group of photon interacting with matter - specifically, the electrons of atoms comprising material slits or material lenses or reflectors. In other words, it is a hybrid energy phenomenon - not pure photon, not pure electron - that is moving through or along the material substrate. When it is doing so, it will travel at less than light speed because it is not true light, but an energy hybrid of some kind.
I fully expect an interference pattern from two different sources, as the experiment Stefan linked to shows. But as it is easy to see, the light from those two different sources is interacting at a material substrate.