Actually I have aided in building 3 rpg's so your are not alone. I would recomend using 3 motors on each rocket for the best effect, and I would also recomend not getting any paint in the barrel, as cheaper spraypaint sets fire very easily, and with a double barreled rocket launcher that would be bad, because you could have the other rocket go off.
Also, an idea for the igniter:
Dont use wire, because then the tube would have to stay connected.
Use a plate, and have the metal part attached to the rocket not attached to the launcher itself, just the rocket tube.
This will keep the tube seperate from the launcher and trigger assembly.
Then have a plate on the rocket launcher that touches the plate on the tube.
That way when the tube is rotated, the two plates will be touching, allowing you to fire the rocket currently lined up, without having the other one going off. Then to chamber the next rocket, simply rotate the tube, and the plate on the other tube will be touching the plate attached to the starter, and the last one would be disconnected.
This also means you could make two seperate rocket tubes (they are replaced upon reloading) be able to drop them in without detaching the last.