you can get this rom hack in smallant's discord server
https://atsignarchives.wordpress.com/2022/07/04/pokemon-emerald-trainer-battles-are-1v2/
Pokemon Emerald – Trainer Battles are 1v2
For this mod, I made every battle (for trainers that have 2+ Pokemon) a double battle, but you only get to use 1 Pokemon at a time. And for double battles, you still only get 1 Pokemon.
atsignarchives.wordpress.com