Answer
$c$
Work Step by Step
We know that the light travels in space at a constant speed of $c$. And we know that the light consists of photons.
So regardless of the two photons' motion with respect to the collision point (the source), the speeds of the two photons will still be $c$ since they are in the same medium (vacuum).
$$\boxed{v_1=v_2=c}$$
