There would be problems to that arrangement. You could add a second barrel submodel, then create another turret entry with the same base and different arms, but it would still be slaved into the main turret base's AI. Both turret barrels would track and fire on the same target. The only way for the slave turret to target independantly would be to add another base submodel for it and make it a separate turret. The downside then comes when the larger turret is destroyed, the smaller one is still sitting there in mid air above the wrecked remains of the larger one. The only way around that effect, would be to give the larger turret, collateral dammage, so when it is destroyed there is sufficient surrounding dammage to take out the smaller turret at the same time. Unfortunately, a collateral dammage features hasn't been coded yet, though it has been requested from time to time.