Plans were drawn up for the Sextuple Mounts, but they weren't considered realistic.
They pretty much did just squeeze two of their three-gun turrets together, though, but the end result was still a six-gun turret - meaning all six guns were independent of each other, unlike the french two-twin system.
>one thing ive heard about big guns is that they have a shorter life the bigger they are.
That is correct, in regards to the barrel at least. The Guns themselves tended to have extremely long lives as long as the parts were properly maintained and replaced as needed - as with any firearm.
>How does this affect naval guns? did the guns of a battleship have to be replaced midwar?
In regards to the barrel, which was the biggest part to change, the American practice (and I believe the British practice as well) was to design the carried ammunition around the expected barrel life of the gun in question.
So, the guns could on average fire one full load of shells before they had to return to a yard and have the barrels replaced. Or, in the case of the Americans, have the yard come to them, see pic related.
So, if you were asking about the Gun Barrels of the Battleships, yes they were replaced during the way.
The guns themselves, however, no. I'm not aware of any Battleship guns that were replaced during the war, even though all of the ships were designed with replacing them in mind if needed. The Americans, for example, took this to an extreme and designed all
of their naval gun turrets/gun-houses of the time to just slide the gun out the front of the turrets/gun-houses without disassembling the turret. This actually did come in use several times during the war with cruiser and destroyer guns, but wasn't used with the Battleships as far as I know.