From the little I've read about the two tier turrets, the theory was that the 12" & 13" guns were slow loading, so parking the faster firing 8" on top wasn't anticipated to be a handicap. Actual performance wasn't so successful - it proved to be an evolutionary dead end. As you say, the two story turret could not be aimed separately, and I believe they used a common hoist for shells and propellant, which sounds a bit challenging by itself. If they used the same size gun in both tiers, wouldn't the loading time for the upper tier be even longer?
It probably depends on why it took longer for the upper turrets to load. If it's simply due to the distances involved, heavier guns would have a lessened disadvantage, because they take longer to load anyways.
The center of gravity issue - was that because the two story turrets were not counterbalanced, at least for the Kearsarge & Kentucky? Would training the turret broadside and the subsequent shift in weight of the barrels cant the ship over a bit, or were there other problems?
Ships want to keep their metacentric height within a certain range, and having turrets and other weight located high above the deck raises the center of gravity and lowers stability. Warships closer to the modern era usually encounter issues with metacentric height when mounting radar and sensor arrays, which can be quite heavy and mounted quite high.
Were there any single tier 3 or 4 gun turrets in that era?
The Italian dreadnought
Dante Alighieri was the first battleship designed with a triple turret (~1909), but the Austro-Hungarian
Tegetthoff class dreadnought
Viribus Unitus of the was the first battleship to enter service with a triple turret, which used a turret originally designed by Skoda for the Imperial Russian
Gangut class dreadnoughts. The French
Normandie class was the first designed with quadruple turrets (~1912), but none were completed as a battleship (
Béarn was completed as an aircraft carrier). In 1937 the French
Dunkerque class became the first battleship with a quadruple turret.
Back to the Brandenburgs: SMS_Viribus_Unitis had written earlier that the mix in calibers was due to the long L40 becoming available later, which I took to mean that the additional barrel length was not an issue on the bow and stern for turrets "A" & "Y". However, the upgraded longer gun length couldn't be accomodated for the "P" turret, with the hull & superstructure format being set.
Turrets A and Y had more room for a longer length gun, as turret P would need at least 55 inches of additional clearance to enable it to fully rotate with the same clearance as before in a single direction, or 110 inches total in additional clearance.
IF they were going to go with six 28cm L40 guns AND maintain the same hull length and superstructure format, I think they might be stuck with a pair of single gun wing turrets, or casemate guns (which could have marginal use in a seaway) What terminology was used to indicate position for wing turrets?
This is a diagram of the turret designations on HMS Dreadnought:
Here are some diagrams for the turret designations used in the Imperial German Navy (they didn't give special labels for fore, center, and aft locations):
By-the-way; I've always found the Brandenburgs to be handsome looking ships too.
Many of the earlier ships have a certain charm to them.