Open it. Take the trigger assembly out (it's purple).

You can then pull the bottom barrel out. Pull the tubing out of it and sand off the glue (shown on the right). The whole assembly can be taken off the end of the plunger tube, too. Take it off and cut the tubing a little above where it comes out (shown on the left).

Then, you can put the second barrel on where the tubing was.

Cut the barrel a little ways down to bypass the air restrictor and hot glue the secondary barrel to the post it's resting on.

Now you need to make the second barrel fit out the front of the gun. I recommend pretty much cutting off the front of the gun. Not quite that drastic, but close. On the left you can see where I have cut. On the right is the half that hasn't been cut yet.

I ended up cutting it like this. Anyway, cut it the way you like, reassemble, and close it up. There is a little increase in power, and it has two shots without reloading. Just switch the little switcher and cock it and you're ready. This gun still sucks tho: get a splitfire and you will see :).