That is not a condition that is considered normal, it can result in things happening that seem strange. I.E. you delete a file, refresh, and the file is still there, because the second instance is now revealed. It's normally not something you do on purpose, typically it's considered an error. If you are interacting with the user share system normally it can never happen.
The fuse system does work down the disks one at a time, the first instance of that file name will be the one presented in /mnt/user. I can't remember exactly what the order is though. Order is not configurable, beyond renaming devices.
Bottom line, you are colouring outside the lines on this one, things may not work as desired.