No no no! Stop the presses!
I've seen a vehicle with two seperate gun turrets!
It can be done!
Go here:
http://ofp.gamezone.cz/index.php?sekce=unofaddons/boats*copy/past that whole line including "/boats"*
and download the "Fast Attack Ship."
They made this pretty average ship, but they used a script to attach a 30mm cannon turret (a seperate object) to the hull. The ship has its own gun turret on the bow and the 30mm turret is only slapped on if you activate the script.
The turret had its own gunner and all the script did was attach it to the ship. There were no really obvious separations as the ship moved around, and I both commanded the ship and manned the gun without problems.
Download the addon, rip it apart and have a look-see...I'm just pointing it out, all this coding stuff is beyond me.