Doesn't quite work that way either.
If you mount both ACD and Gdrive and let's say ACD falls off the map (gets unmounted) then yes, it would pick up the Gdrive files. But if for example the files are still present in the mount, but can't be read / issues reading / slowness, then it will not go to read them off Gdrive.
A good example is a typically Google Ban wihen using rclone and Plex. If you mounted a UnionFS with Google first and ACD second and you got banned from Gogole, the files are still present (they just can't be read/downloaded) so whatevever is trying to access the files would not be able to, as UnionFS would still present those files, as they are being presented to UnionFS via the rclone mount point.
If you were to kill the rclone mount point, unmount the folder, then the reads would happen via ACD.
If you want to get really into the weeds and technical with scripting, I guess you could have a script check every X interval to see if it can read/download a file from Google Drive, if it fails, unmount and let ACD take over.