1) Savefiles are found in AppData/Local/<name you saved the GM project under>, monster_save0 is the first file, monster_save1 the second and so on. It's a hidden folder so the easiest way to find it is to type in %LOCALAPPDATA% in the file explorer path and hit enter.
2) The important part is setting global.player_side_monsters and global.enemy_side_monsters to 2 (that's what controls how many monsters each side is allowed to have at once). The easiest way to inject this would be to copy obj_npc_trainer and cc_battlestart_trainer to new obj_npc_doublebattletrainer/cc_battlestart_doublebattletrainer which are mostly identical but sets these to 2 instead of 1 before the fight starts. (Also note that cc_battlestart_trainer actually can take an array of trainer data, global.encounter_trainer_data has one entry per trainer, so if you want a double battle against specifically two different trainers (instead of an entity like 'twins') you could populate this twice with different monster, dialogue etc data for each trainer.
3) Use an obj_encounterzoneselector, place one in the room and stretch it out over the area you want the battles in and then select the ects_* script with that room's settings (there's two more ects_* scripts with the first town / Driftwood Forest encounter data). They're tied to these markers so you can make things more granular (e.g. have different encounters in different terrain)
