Okay, so here’s the thing — when we talk about photons and their "relative velocity," we have to remember that in physics, especially relativity, things can be a little sneaky compared to what our intuition says.  

We know a photon always moves at the speed of light c in any inertial reference frame. But if you naively think “two photons coming toward each other” like cars on a highway, you might be tempted to say “that’s 2c.” The problem is… relativity doesn’t let you just add speeds like that. The way velocities combine takes into account the fact that nothing (with mass or energy) can exceed c from any frame’s perspective. So even if they’re approaching head-on, each photon still “sees” the other moving at c, not faster.  

Basically — the relative velocity here, by the special relativity definition, is c.  

Do you want me to walk you through the velocity addition formula, so you can see how that “2c” idea breaks down mathematically?