Ok, I finally figured out how its supposed to attach in battle station mode. Again, this is really weak. It attaches weakly via two tabs on the bottom of the gun and attaches over a spot that appears to be made for this. However, as I stated above, it's a really weak connection spot that again appears to be an after thought. In addition, I now have a small paint chip on Metroplex thanks to connecting one of the guns in battle station mode.
I'll stand by my original statement that Metroplex appears to have been designed with one gun in mind and not two.
The instructions in the SDCC version do not show how to attach the guns in city or battle station vehicle mode. I'll update the gallery in the next 30 minutes.