Each shop can have only one recurring charge per app. When a new recurring application charge is activated for a shop that already has one, the existing recurring charge is canceled and replaced by the new charge. The new recurring charge is then activated.
FYI to anyone using RecurringApplicationCharge 2021-10:
When creating a new recurring application charge, the old one is not cancelled.
I just discovered it when testing with 2021-10. I was able to create multiple recurring application charges on the same store with the same app, (test": true) but each one with "status": "active". This is not what the current docs say it should do. So this is warning to anyone using this API.