Funny you should say that 101181920, because we were just talking about weapons of the same name, but that's not how the work, unfortunately. The reason that outfits fire the same weapon is because they link within the outfit resource to the same wëap resource. For example, you could have (assuming they were all available) three different instances of "Wraith Cannon" in your secondary list, but they would not fire together.
While you can't get exactly this effect (having it as a secondary weapon makes it harder), here's what I would recommend. Render the image for the missiles, but render it with a lot of extra black space around it. Then, rather than have the missile in the center of the spin, have it placed in some corner. I may be wrong about this, but I believe that the AI always thinks that a particular object, be it a ship or a weapon, is placed in the very middle of that object's spďn. Therefore, if it tries to fire at the incoming missile, the shots will largely go right past it, through the black space in the spďn. You could even have the missile move around in the black a bit in the spin animation (so if it's facing 0° it's in the upper-right corner, but if it's facing 90° it's in the bottom).
Of course, this is a double-edged sword (if it even works). If fired at a smaller ship, there's a chance the missile will blow right past since the middle of the missile's spin will seek out the middle of the ship's, and the actual physical missile will miss it.
Like I said, I'm not even sure if that's where the AI aims for, but if it is, that would be a method of simulating what you described. As for actual decoys, I think that's impossible (I entertained the same notion a while ago but never found a way to do it, either).