I was looking for a way to do this as well. There is one solution, it’s not perfect but works.
You can use rclone to do this. It lets you copy and sync files between cloud services. You simply copy the files from one service to another, or local to the cloud.
Limitations are that if you are using cloud only backups then you have to re-download all changed files before uploading them again, and that any corruption to the primary backup set will propagate to the other.