Looks like the big brother of AMOS, prob. for the same reasoning.
And whats the issue with simult. firing? Two barrels would just double the rate of fire at some increase in weight.
Less exposure to counter fire...
Some guided munition and they land apart from each other, not in the same spot.
If they fire simultaneously you have a major recoil problem.
As for increasing the rate of fire, you probably do not want to be in any part of the loading process for one barrel when the other one is fired. Any misalignment and everything smashes to a halt, if you are lucky. Smaller guns can handle this because they used fixed ammunition (projectile and powder in a casing are a single unit, like a rifle cartridge) and the lesser size and weight of the ammunition permit a larger (relative to the size of the round), and more rigid, ammunition handling and loading mechanism. All current 152mm/155mm howitzers use separate ammunition because of the difficulty of manually loading 140+lb rounds some 4 feet in length.
I suppose you could load both barrels simultaneously using a preloaded clip and semi-fixed ammunition. The practicality of this approach would depend where the bottlenecks are in the current loading processes for 152mm/155mm howitzers and the handling of the spent propellant casings. You would have to prepare and load 2 rounds into the clip, which would then be moved into position and then both rounds simultaneously rammed. However, ammunition storage space would probably increase by 60% to accommodate the propellant casings, and clearing jams and misfires would be extremely difficult. The length of the semi-fixed ammunition is also over 2x that of the components of separate ammunition, increasing the size of the loading and ramming mechanism by as much.
On the other hand, just going to a semi-fixed alone with a single barrel could increase the rate of fire by 60% to 80%, until cooling the gun barrel became a problem after 10 to 12 rounds. And probably weight a lot less.