Serebii just posted this:
During the final episode of Pok?mon Smash, it was finally fully showcased how Mega Evolutions work in battle, answering a question that has been in place in some parts of the fandom since the revelation. It proved that you can have a Mega Evolution in combat if your opponent also has one, with this footage being the first showcase of two Mega Evolutions being seen in battle, as seen in this battle where Mega Ampharos was used against Mega Blaziken confirming that it is one per trainer in battle, not one overall per battle.