I was wondering this myself. I currently have a 722 and a 722k. I want to keep two, separate tuners/DVRs, but Dish fights me on this. They say I have to have one Hopper and a Joey. I don't want to share one DVR.
Use two Wally receivers, and add your own external hard drives. There is no way to share between them that way.
The drawback, of course, is that Wally will not work with any of your existing recordings, if you were planning to transfer those. In that case, you would probably want a Hopper Duo and a Wally. Just keep in mind that the Hopper Duo (or any other model Hopper) would be the
only receiver on your account capable of playing archived recordings from
both of your 722's.