I'm of the opinion that if such a thing were to happen, the game might just crash. Especially if the FoH has JUST landed, a millisecond before the Knee of Doom hits. The explosive impulse and the concussive force generated by the two would be... potentially terrifying, if it were to actually have an in-game reaction beyond both dying and anyone caught in the FoH radius dying.
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Dude, it's called the Knee of Justice [spoiler]Super Smash reference[/spoiler]
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The Destiny Universe would implode. Like if two Nova bombs hit each other