Okay I tried this again and I finally figured out how to do it 100% of the time. Follow the directions in this exact order:
1) Select the lightning gun
2) Pause the game
3) Hold down the L trigger
4) Hold down the side gun with your left thumb.
5) Unpause the game
And that´s it! Now you have lightning fire 100% of the time and whenever you press fire you have side guns. Even if you die you´ll still fire lightning.
You can switch between all the weapons, but once you select lightning again the glitch will stop.
This opens up a slight bit of abuse since you can kill things twice as fast now with lightning and a second forward gun...
- cough* chuuroh city midbosses ** cough ** zenshoh blimps **