Run Dragon and type 1 for PCjr.

The game takes a little while to load, so don't worry if you see a black screen for a bit. Just look for the second title screen (the Double Dragon logo in red, orange, green and black cga colors) and hit the joystick 2 fire button for a one player game.

One oddity of note: joystick 2 controls the first player. So if you start a game by pressing the joystick 1 button, you'll start with a two player game.