The top button replaces the "Yellow" Jump button, and the bottom button is the "Blue" Pogo button.
To fire, you need to press the button on the keyboard that was assigned for fire (usually the "Green" Spacebar), use Gravis Gamepad mode, or turn on two button firing. If you turn on two button firing, two things will happen.
To fire with two button firing, you must press BOTH the jump and pogo buttons at the same time.
With two button firing, the "Impossible Pogo Trick" truly is impossible, as you would fire your blaster instead of doing the trick.