I have no doubt these are rare. I wonder how hard it would be to construct one? http://www.gamesetwatch.com/2006/01/alpha_dreams_homebrew_ds_hilar.php I want one!
No I don't think it is a joke. They needed something "DS" like to test the games. If that picture is not fake you can clearly see the game streching over the two screens. Also they needed more buttons to simulate a DS experience.
I kinda thought it was real, but used as a tech demo just to show how 2 screens would work. I don't think they actually used it to dev games. As for how to make one, it's easy: Take a GBASP solder wires to the pads for the controls. Connect the wires to a SNES controller. Glue the controller to the gbasp Glue a GBA to the back of the gbasp Write a game for the gba that uses one system to control the screen of both systems over the link cable. Done. ??? Profit I could probably make one in an afternoon, minus the game programming.