Select the "Gallery" option from the main menu, then quickly press B, X, Y, B(2), Y, X, B at the first screen to unlock the Stranger jet and all gallery art. If you entered the code correctly, you will hear a sound. Alternately, successfully complete the game on any difficulty setting without using any continues.
Select two player mode, and have player one select a jet. Then, press Start + A on controller two when selecting a jet for player two.