Unless your cable box has the capability to record TWO or more programs simultaneously, then I don't think you can do what you are requesting.
The alternative would be to rent a second cable box from your service provider, connect one box to the SB, and the other box to the TV.
In my case, my TV service provider provides about 50 channels without a cable box, and about 100 additional channels if a cable box is rented. So I put a splitter on my incoming TV signal, run one feed directly to the TV to view those first 50 channels, and the other feed to the rented cable box, and from there to the SB, and from there to the "video 2" input on my TV.
So long as the folks sitting in front of the TV are satisfied to watch those first 50 channels, then I can record and sling the larger set of channels through the cable box-SB pathway.
But when they want to want the additional channels, I have to stop slinging, otherwise, as you noted, whatever I do via the SB is mimicked on their TV screen if they are viewing signals through the cable box.
This is what the SB website says about the PRO-HD in its FAQ.
It doesn't tell you how to set it up though.
- What happens if I am using the Slingbox and someone at home is watching TV at the same time?
In most situations, both of you are watching and controlling the same device. However, the Slingbox PRO-HD includes an internal tuner, so for standard cable (coaxial) inputs, you can set up your Slingbox to watch your TV, DVR, or other A/V source while the remote user watches standard cable (or vice versa). Also, many customers connect the Slingbox to a secondary TV source in another room to avoid remote conflicts.
This statement is correct, but pre-supposes you get your cable TV direct from the wall to the TV without using a Cable box, or you use an Over-the-Air aerial.
Just think of the Slingbox as a TV that happens to send it's output down the Internet instead of to a screen. So in your house, if you can simply buy another TV and plug the coax into the wall in your bedroom and watch a different channel to the TV in the main room then you can do the same with the Slingbox.
But if you would have to get a second cable box for the new TV then you will have to do the same with the Slingbox. Or 'share' the main cable box and just accept that you cannot watch different channels.
Make sense ?
With regard to your question about taking advantage of the SB-HD's internal tuner as a second channel-controlling method in addition to your cable box, I believe the answer depends on how your TV signal is delivered at your USA home. I have a different TV provider company than yours, who used to deliver all of the channels I was receiving as analog, with no need for a cable box of any kind. But a couple years ago, they converted to an "all-digital" transmission format, so that everyone who wanted to watch all of the stations as before had to install a "low-frills" or "no-frills" cable box (a.k.a. Digital Transport Adapter) between the wall and the TV, in order to see all of the channels that they used to watch. At the same time, they began to offer many channels in HD, and encouraged folks to upgrade their service with an HD cable box (the low-frills and no-frills boxes only display standard definition). While I chose not to do the HD upgrade for the time being, I did learn that, per federal edict, the cable company is required to make SOME HD channels available on their input line even without an HD cable box, or any box at all -- these include the local affiliates for national networks, plus several home shopping and religious channels, and some local government channels.
So, by installing a signal splitter at my wall inlet, and running one wire to my cable box (and then S-video or composite cable from the cable box to the SB and from there to the TV) and the second wire direct to the coax input connector on my SB, I have two signals available for slinging. One is all of the channels coming thru, and controlled by, the cable box. The other is that limited set of local HD channels, and home shopping/religious channels that I can pick up with a direct cable connection between wall and SB.
So, it is possible for me, with some limitations as noted above on what I can sling at any moment, to sling some of my channels through the internal tuner of the SB while in-room people watch all of the channels coming through the cable box.