Nope, my guess is that the older gen (pre-922) VIP's won't do it because they will want people to upgrade to the 922 even though that without Harmony 1 or normal remote control (their dumb touchpad interface) I think the acceptance will be very slow.
All slingbox inputs are passthru's. If you look at the back of any Slingbox (I have an Pro HD and a Solo) you'll see that there is one line of inputs and right below it one line of exactly the same connector which are outputs. So a Slingbox literally goes 'inline' with your cables and doesn't rob you of a thing including the Antenna connector.
You'll only need a SlingCatcher if you want to watch the other DVR on your TV. But then, if you wanted to do that, why not just use the modulated output and tune to the channel the DVR is on. Any modulated dual DVR has a UHF remote so you can watch it anywhere.
If you do want to hook up a Slingbox you would do it because you'd like to watch your DVR's on your computer either in the house or while you are traveling. I travel a bit so I installed 2 slingboxes on 2 of my 3 DVR's. So from any hotel room (or boring meeting ) I can watch the latest version of House or Doctor Who whenever I want!.
It robs you of control. What the OP and I would like to do is have true MRV like DirecTV has. No 922 or Slingcatcher necessary. No remote control of another family member's receiver. And preferably no added expense, such as a 922 upgrade or a Slingcatcher. IIRC, the DirecTV MRV feature even allows you to view digital content via the DirecTV receiver. (I just bought an Asus O!Play, which would not have been necessary had I an HR20 rather than a 722 and 2 612 Dish receivers.)