You'll need to run a second line from the dish to the 612. The 612 is a dual-tuner receiver and will need a separator at the back of the unit to split that single coax into the two tuner inputs.
You might reconsider the Hopper 3 and one Joey. You pay $10/mo for the 722 and the 612 would would add another $10/mo. Hopper 3's "DVR" fee is $15 plus $7 for the Joey so you'd pay $2/mo more but say good-bye to tuner conflicts and you'd have a common pool of DVR content.
The other advantage of this setup is it only requires one line from the dish and that could be split more or less anywhere using a "hub" go get the feed for the Joey.
There are those that prefer to keep content separate (spouse and/or children) so H3/Joey might not be for you.
Contact DIRT (in the helpers section in red) to see if you can get a free upgrade (would be leased and a 2-year contract). Then you can sell the 612 (and the 722 if you own it). PM ("private conversation") them your account number or phone number plus you 4-digit security code and they will get back to you with your options.