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How to set up recordings on second Hopper from Main Hopper?

Wouldn't both Hoppers need sling? I have one Hopper with sling, and one regular Hopper.

Sling is not needed to set recordings, only for remote viewing. I'm not even sure if you need the receivers connected to the internet to set recordings. I believe it just does it through the satellite signal.
 
An extra Joey is cheaper than a mobile device...

Only for a few months, and thereafter you might as well buy the mobile device and avoid the $7/mo charge that you get with a Joey in perpetuity.