At the title screen hold Right and B on Controller 1 and hold Left and A on Controller 2. Press C on Controller 2 with everything else still held down. Now both players can be the same person in 2 Player.
More Lives
Go to the main menu. Hold down (A) + (B) on controller 2 while selecting Options. You can now select how many lives you start with (up to 9), choose your starting stage, and play at the Very Easy and Mania difficulty levels.