In my early N900-days when I played with the TV cable and my TV I got a call just when I had my N900 screen on the TV. Usually the screen should turn off during a call but the TV screen didn't turn black.
Additionally the output can be switched between PAL and NTSC and both produce slightly different images on the TV screen (my TV can handle both).

So I'm pretty sure that the TV out is handled seperately. But the question remains if the GPU can handle more than one display. I can't check right now, do you see any useful output in Easy Debian when you connect your N900 to the TV and issue the following in a Debian terminal: