Okay, this kept me awake all night (or was it all morning...).
I figure that if it did fire from an open bolt, fixed firing pin style, and when you pull the trigger all that did was release the lower bolt, firing the lower barrel. But the top gun is regular closed bolt, you don't chamber it first shot, the gun does all by itself, when the lower barrels gas pushes the bolt back. So then, if you keep the trigger held down, the upper bolt goes back, feds from the top mag the bullet, and then fires the bullet, because the top gun has a hammer and free-floated firing pin, but the lower "slam-fires" everytime you pull the trigger. So then the gas from the upper barrel goes down and cycles the bolt on the lower gun, feeding another round and firing it. This cycle keeps on going, seems it would be hard to stop it. Maybe the lower bolt wont close if you're not pulling the trigger.
Or maybe it fires both barrels at the same time, then operates as a normal gas system rifle would, just firing two shots at once, instead of just one. If that was so, the gas tubes criss-crossing would just be an unnessecary complication, and could lead to a really bad jam, because if a jam occurs in just one of the barrels, it would cause the whole gun to stop functioning. So really it makes the gun twice as likely to jam.
But it's a neat idea.
Am I right on the operation system?