Tip for others because the double battle option should be something you have to turn on if you ask me.. but if both joy cons are on I guess it spawns another trainer to double battle with you, and use your pokemon.
All you have to do is go into the options and change the battle style to "set" if you want to play solo. Maybe it'll actually be more fun playing solo.. I don't really like the pokemon go style catching system, but battles don't seem to be about size as much as pokemon go so maybe the slightly changed story, and enhanced graphics will keep me playing through it..