You would need to use ev_as and do some scripting in order to make that work. We have some tutorials on the website here. A good example of a double battle that you can look at would be Lucian's battle right outside of Solaceon Town. It'll be in t04.ev when you get everything from the link set up.
#asking how to make double battle mod
shrewd osprey
sullen oyster
@shrewd osprey Can I ask more question?
shrewd osprey
Yeah you can ask them here. I'll get back to them later since I'm heading to bed right now.
shrewd osprey
You would need to use ev_as and do some scripting in order to make that work. We...
romfs\Data\StreamingAssets\AssetAssistant\DPR\ev_script. Read the setup tasks here and don't ping me. I'll get back to this when I have time