Hope this helps.
You can have as many drives as you want, as long as they meet the requirements of size and being powered.
You can transfer a recording to any one drive. When on that external drive, when that drive is plugged in the content will play from that external drive.
You can move, never copy from the Dish receiver to a drive, or back from the drive to the receiver, which would then allow it to be transferred again to a different drive or back to the same drive if desired.
To see the shows on the drive that is plugged in there is a selection on the Dish receiver that shows everything on the external drive. You can also put pictures on there to be viewed. I notice you keep using the term "Backing up" which is not correct. You are moving a file, and there will not be more than one instance of that file/program.
The EHS is really for keeping shows that would otherwise fill up the hard disk on the receiver. A by product of that is the EHD is safer and hold better hope of the show being available to be watched at a later date.
One drive at a time can be actively plugged in.
Most people who have posted would recommend you to put important shows onto an external drive, as that has a much better chance of surviving than a receiver drive, and as you know, if the receiver crashes.
Anything more, like mirroring files to an additional drive is beyond the normal use but technically can be done. You would not be able to play those mirrored files however as a second set of saved shows.