On my former cable box I had the ability to view/stream a channel while browsing the guide. There is another thread for this I think, so I voted on it there. But I could select a second channel and have both channels stream at the same time. It would place the images side by side, or one channel in the full view with a PIP inset of the other channel that you could swap back and forth.
Only the active window would be heard...but the fact that I liked was that both channels were streaming so you could go to either channel and not loose the buffer.
I had a similar device years ago that let me bring up 4 screens...all simultaneously buffering and you could toggle between each one and not lose content if you needed to rewind. This was great for sports--even just a 2-window concept, where you could watch/listen to one channel while you waited for the commercials or commentary to pass, etc.
I know I can just hit record but then have to go back and scan through what I may have wanted to see, etc.
Having at least 2 video windows simultaneous stream would be the same as watching live what you are recording, if even that was possible. Again this was used by me mostly in watching a game and waiting for something to happen I would watch a recording or another channel at the same time.