Now, I may be incorrect, but I don't believe that you will find the software to do what you wish. I would imagine that this is a hardware issue.
Some video cards will allow you to view both, and others will allow one at a time.
If you have to, rather than run the TV off of the S-video cable, you can run both of them off of the standard SVGA plug (i am assuming that you have a desktop-not a laptop).
You can buy a splitter and a scan converter. A scan converter will just convert the SVGA signal from your video card to NTSC for TV. You can get them with S-video....
The splitter just connects to your video card. One side of the splitter cable will go to your monitor, while the other one goes into the scan converter. Go from the scan converter to your TV.
--or--
If you have a little TV laying around with an s-video plug, simply split off of the s-video from the video card and set the little TV next to your monitor
--or--
If you don't like the price on a scan converter, then you might look specifically for a video card that will handle both outputs at once!
Anybody out there have another idea? ----------------------------------------
Groucho Marx said it best...
"A child of five could understand this. Fetch me a child of five."