Not exactly a tournament (or even a single battle for that), but in Multi battles, one can actually have 2 Mega Pokemon on one side of the team, as all players are allowed to choose one Mega from their teams. Now if the two players were to coorinate or something you have two Megas that are ready to open fire.
Other than that though, no there is no official way of having two Mega Pokemon on one team.
Hope I helped!