You'll need a compatible powered usb hub that will be able to tell the difference between the two, but as Matthias said, it may confuse the box unless you are making sure all of the mounts are setup correctly.
If you're already using a powered external hdd though, why not just use partitions? This is how your regular computer runs both an OS and media files on the same drive.
If you're already using a powered external hdd though, why not just use partitions? This is how your regular computer runs both an OS and media files on the same drive.