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They're almost certainly not, it's done by whichever kill is registered first (I've won 30 - 30 games as Empire). I don't know how they handle two players ramming into each other for the final kill though, it could be random, it could be the person with the higher health dies 2nd, or it could be something horrible like network latency.
Whichever death happened first would trigger the end, which in the case of a MAD joust will generally be the one that was at lower hull health at the time of collision, though there are enough variables in the collision dmg calculation that that’s not guaranteed.
In the exceedingly unlikely event that both deaths were confirmed on the exact same frame then the game would actually wait for another kill before deciding the winning team, just like we do if the teams are tied when the timer runs out.