Just thought I'd mention this here; If you put 2 video streams in an MKV and open it in VLC it opens two windows, one for each stream. Then you could make the gap however big you want :)
Upsides? For a start, if you have a screen which isn't doing much at all then it's a huge decrease in filesize since that video stream can have a really low bitrate (and extreme decimation if the encoder uses this). Each screen can also be resized independantly and moved to where the viewer wants.
Downsides?
Well, it doesn't do scaling gracefully (it does nearest neighbour for some reason) and so anything that needs aspect ratio correction looks bad. Luckily for DS games though this isn't required :) Correction: Using OpenGL for rendering no longer leaves me with this problem. Also the implementation is a big buggy at the moment..
I tried a proof of concept about a month ago with a couple of the runs I was encoding at the time. Here's the result:
Edit: I'd just like to mention theres a whole load of other uses this has as well for other consoles. You could have a display of memory values for those that want them or a special visualisation. Even input and frame stuff could be put in a seperate stream at little space cost.