Well, the relative speed between two photons, if they come from a coherent light source, would probably be close to zero. However, if you have two photons going in opposite directions, the relative speeds would be close to the speed of light. Either way, it would be between 0 and the speed of light, inclusive.

However, technically, no reference frame really makes sense at exactly the speed of light, because the definitions of time and velocity and many other quantities just break down, but for the sake of trying to answer your question, just know the relative velocity has to be between 0 and the speed of light, and the same goes for between any other two frames.

Technically, no reference frame really makes sense at exactly the speed of light, because the definitions of time and velocity and many other quantities just break down, but for the sake of trying to answer your question, just know the relative velocity has to be between 0 and the speed of light, and the same goes for between any other two frames.