Here's a solution with kindoddball's proposal without odd scaling.

N900= 800x480
DSi= 2x (256 x 192)

So have 1 screen doubled (512x384) and keep the other one original (256x192).

Pushing them out to the corners allows a blank space to distinguish the individual displays and allows you to be able to concentrate on a single screen much more conviniently. There's more space to put the Applications commands (File, Tools etc). There's even more space to place information like fps, shortcut keys (as demonstrated), other keys (save state, load state, toggle keys, sound on/off) or a combination depending on the builder.

Here are some quick scetches for understanding:

First Image Red biased

Second Image Blue biased

1) Based on the exact pixels as stated above.
2) Red= main screen vs Blue= touch screen
3) Star button toggles between which screen should become larger (ie doubled). Useful for some games eg New Super Mario DS (red biased) vs Drawn to Life (blue biased).

Edit: Images published on imageshack won't automatically display on forum, now I've just made them as links. So you'll just need to open both of them in individual tabs, sorry.