This will only work on split screen.

Go to split screen and go to free-for-all or team death match. Then choose Crash.

Pick any gun then go to the the big building behind the crash. Go to the main top and fall off of it. Then die and pick a gun with second stance.

Then do the same thing and drop at the same place you did before. Right before you hit the ground press X then you will have the gun you died before with on second stance.