That's intresting to say the least, being there's so many options for sound for the 7800. Ask for XM project, I'd like to think some programmers may implement Stereo for a whole new dimension of classic Atari sounds.
That being said looking at the original schematic for the 7800 the two audio lines from PINs 12 and 13 from the TIA do join quickly and then with PIN 18 from the cart port which would be POKEY I believe joins alittle down the line.
I'm just curious if they are the same audio signal before they get joined or are they "noise 0" and "noise 1" or "noise" and "music" channels. Because if they are, which they should be, then one could seperate them by connecing straight to the pins and have different sounds on two speakers. Of course games without music would benefit the most since you would have different sounds coming from different speakers. Kind of like the 2600 was supposed to do by having two speakers in the case.
Now with the upcoming XM, one could use the two channels of audio from the TIA and have them as seperate left and right noises while the YM2151 can be music mixed in Mono or Stereo. And why not add a center channel using the POKEY, so 5 channels of audio to work with could easily be turned into a stereo mix. Just brainstorming there alittle bit, just to hear that Missile Command sound go from speaker to speaker in a Stereo mix would be kind of cool.