This doesn't work for light because photons have zero rest-mass. As in, one photon cannot be considered to be at rest relative to the other, because they can never be at rest.
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But what about the 2 objects moving towards each other at/near the speed of light[spoiler]sorry for all this theoretical stuff[/spoiler]
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It's not theoretical, they just have a high velocity collision. That's what the Large Hadron Collider does. It is a [b]large[/b] ring that speeds up [b]hadrons[/b] and watches them [b]collide[/b]. It's helpful for studying particle physics, the history of the universe, and confirming theoretical presumptions, like the Higg's boson.