…That’s my point. I’m pretty sure the Dynamax band works per battle. It’s the same reason why you've never seen somebody Mega Evolve another Pokémon after using another.
It's a game rule, but was it ever mentioned in the show?
During XY, both Sawyer and Alain had 2 Pokemon who could mega (Salamence & Sceptile/Metagross & Charizard), but the situations where they used both only came up during the Kalos League. I don't think anyone else relevant had multiple mega-possible Pokemon + Mega Stones weren't exactly easily obtainable in the show (unless you're Audino but we don't talk about that episode). My in-universe headcanon says the Kalos League might have had rules against multiple Megas, but the real reason probably was to emulate the games. That being said, I don't want to watch all of XY again to figure out if they explicitly said anything.
Z-Moves very evidently have that restriction so nothing to say there.
PWC so far always had that 1 gimmick restriction and since I only primarily watched PWC fights, idk if anything was said about being able to DMax/GMax multiple times outside of it. Someone feel free to correct me.
TL;DR: unless anyone wants to go back and find a clip of someone explicitly mentioning 1 per battle for DMax, I still think it's a possibility. + As you said, it would be unfair if Leon only has 2; I'm down for the anime to break the game rules to balance things out.