What happens when 2 different players have Mecha'thun die at the same time?
If both players have their Mecha'thun die at the same time, it will actually result in a draw.
Usually it is true that the first deathrattle minion that was played resolves first, but win conditions are not checked between deathrattles. So the game registers both players as dead once everything resolved, resulting in a draw.
Video of it happening