I've been trying to do the same for quite a while. I've done some research and tried and failed. If you only have one graphics card, it won't work properly. I don't know why. I got it to work using xdm (If I remember correctly), but there is a bunch of bugs as long as you use tv-out/svid on the same card as vga for your monitor.
The bug I had was some rendering errors, and as fast as one of the users loged out it locked up. The solution seems to be a second VGA card for your TV. I really wanted this to work, so I bought a new nvidia card. It turns out my motherboard got a bug, so it doesn't detect my second card