Before you read the rest of this post keep this in mind: I have not tried to duplicate this glitch and it may not be duplicatable.

I was playing some Halo 4 last night, hijacked my friend's Banshee and it was rapidly switching weapons. I thought nothing of this until I actually shot, when I shot I was shooting a plasma cannon and fuel rods at the same time. It would be great if someone could test this out in a custom game, I can't right now but plan to when I get home.
According to my friend he was switching weapons back and forth as I hijacked him, so I believe what you would need is an enemy flying a banshee down near the ground that you can hijack. As soon as you jump on the banshee and begin hijacking it your friend needs to swap between the plasma cannon and his fuel rods.

I do not know for sure that this is duplicatable, like I said, but I thought it was worth a try.

Update on if you guys could duplicate it or not.