When you first turn the game on, simply press GAME SELECT several times, hold down GAME RESET.
Switch the console off and then back on again then release GAME RESET.
This should give you double fire instead of one at a time.
At the main menu, choose either one or two players and press Circle (instead of X) to begin the game. An option screen that allows anylevel, including planet Mercury with a new power up and an "Alien World" featuring the best background in the game.
Enter the "Level select" code, choose level 00, and complete it to unlock an ending, classic mode, and credits. Press Right when choosing then number of players to access classic mode. Enter the options screen to access the credits.