This gun set is basically a hybrid of design elements from Red Impact and KillerK's gun. The only original, 'innovative' feature here is that they pull each other. Hopefully I won't get too much flame for putting KillerK's firing system to some creative use. :)
Pieces that will need to be modified/mutilated:
4x Dark Grey Connector (will be 'decapitated')
2x Tan Lock (will be weakened)
Step 1: Create Specialized Pieces.
No, we're not damaging K'nex parts; we're ENHANCING K'nex parts.
Find a safe way to 'decapitate' 4 dark-grey connectors by cutting off the top halves of their loops. I stuck all of them on a rod and used a small hand saw. Makers of my Red Impact should already be familiar with these.
Also, we need to 'weaken' a pair of tan lock connectors so that they do not cause as much sliding friction on the firing pin they will be attached to. I have found an easy way of doing this that involves putting them on the end of a rod and turning them carefully. This permanently weakens the plastic and they'll never be tight again. I would mark them with a sharpie.
Step 2: The Firing Pin's Shaft and Trigger. (Rear)
This is the rear section of the first gun. The tan lock in this step is one of the two that you have weakened (clearly, you're saving the other for gun number two). It should be simple enough to build based on these three angles, and note the third one shows where the auto-lock block trigger's rubber band belongs. The black rod should have a very easy time sliding through.
Step 3: The Handle - Middle Section.
The handle section is built similarly to the Red Impact's. The orange-colored rod is a bendable one. It is there for comfort (my hands started really hurting from rubbing against the bottomside of the back end, and this smooth rod in the way fixed that). If you don't have a bendy rod, I still highly reccomend a yellow rod there.
The camera flash was being kind of weird, and I was being kind of lazy.
Step 4: Ammunition Hopper - Front Section.
The inside of this section uses two decapitated dark grey connectors.
I'm still not used to my new macro close-up lens. The lens itself is physically in the way of the camera's flash light so when I'm too close, this happens. -_-
Step 5: Attach Segments Together and Add Structural Brace.
All parts previously assembled (and a small new one) are to be added together as shown.
Step 6: Build an Identical, Symmetrical Mirror-image Copy.
This is DUAL pistols, after all. Since the triggers are on the sides for thumbs, one must have its trigger on the other side, which means its structural brace must also be on the opposite side from the first. The second one should go much quicker, now that you have one already built to use as reference.
Do they have to match by color? No.
Step 7: Assign Genders to Firing Pins and Arm Them.
One must be male, and the other shall be female. *snickersnicker*
As with nearly all firing pins of this type, it will violently explode if you don't use tape. Tape stops this by refusing to let the connectors widen up and detach. The best tapes are ones that do not stretch, like packaging tape or (I reccomend) alluminum tape. Wrap at least 3x around to be safe.
The firing pins must be powered by rubber bands. The anchor points for the rubber bands are are (opposite of the trigger side) the Structural Brace and (on trigger side) the blue rod of the handle. Take two rubber bands, bind them up, and stuff their combining knot into the firing pin's dark grey connector.
Step 8: Loading/Firing.
The hoppers are loaded from the front. They are designed with the backs wider so the rods can take places beside one another. This allows 7 to just barely squeeze in.
The intro video explains it best, but the way to very quickly pull back both firing pins at once is to hook the male into the female ends and pull on both. Be sure to hold them straight, facing perfectly away from each other. After the triggers auto-lock, separate them, aim, fire.