I haven't found a way yet but our 622 is in single mode most of the time. I think there was a thread recently where somebody was talking about some kind of converter for TV2 IR, don't know what the result of that was.
I don't have a sling box, but I do something similar with my location free on a 625. The 'trick' is to use the TV1 with the IR remote and an IR blaster. Since TV2 is a uhf remote my location free (and probably your sling box) will not be able to deal.
The problem of course is the HD output is only on TV1 on the 622, so you would need to either always be in single mode, or switch to dual mode when you want to watch a SD program on TV2 and wire TV2 to your SD inputs on the tv and TV1 HDMI to the TV and the non-HD outputs to the slingbox.
This is exactly the reason that I got both a 622 and a 625.
yea I got it running, I had to put tv 2 with shared view enabled.. And its running in single mode. (unfortuanly, If my wife is watching tv, she will have to deal with it or watch tv in the other room or fight me for channels ) Love to see how that one would turn out LOL .