>>55087696ROWE emerald is a compfag romhack that has an option to turn on only double battles when you start a new file.
However the AI is bad at doubles battle + teams are made up of pokemon built around singles.
What results are battles with tough mons but not overwhelming. All pokemon are catchable up to sword and shield DLC.
The catch is that it is an open world romhack which is not for everybody.
This is a pro in that you can roam the map early with all HMs unlocked to find the pokemon you want, but it does remove the storyline and team aqua/magma subplot.
It's just pokemon battles really.