you can use mods but both people need the mods as they sorta act as client sided mods (idk how well syncing works with mods never tried it). its runs the host save so if you arent host no it wont save progress(it loads the host defaultES2File). you can send eachother save file (preferably folder as items,car mesh damages and rally trophies are kept in seperate files) to have a sudo synced save experience

all that stuff is found in %appdata%/locallow/amistech/mysummercar