Sling Loaded is only for the yet to be released 922 (at this rate, released by Jan 2010?). It is not available on any of the current Dish boxes. You may use an external sling unit, but I don't know the details of that and if you want it in HD, it can be costly.

Or, if you have ViP boxes, you can get an external hard drive, have that feature activated for a one-time fee, then you can transfer the recorded program from the original DVR to the external HDD. Once that is complete, you then connect the external HDD to the other ViP box in the other room, and playback the recording from the external HDD. You can also transfer the recording from the external HDD to the other ViP box, as well.

I do this for the movies I record when we all want to watch them on the big Sony in the living room. I record and transfer to HDD all my movies from the box in the office.