WARNING: REALLY LONG REPLY
Nice idea. Have you tried to "bash" some sections together with wood, or cardboard, to get a 3 dimensional view?
No. I’m not that skilled. (In fact, I suck.)
Andy R, that's really a great idea, indeed I was thinking about such things too for a while! I think even the "waterproof"-problem is not a real hard one (even if I have not included them in these pics, but if you are interested in my solution I can post them too.)
The only “problem” here is there are no hatches. ¿How are you going to access the below-deck areas without them? (And they need to be a bit bigger to let hands in.)
There is no apparent notch to insert the mast in, and no apparent support for the masts… The second part is a minor one, but there needs to be something to put the mast itself into…
You did, however, potentially figure out how to solve my buoyancy/stability problem in a most inventive way- I’m not sure if you meant to do so or not, but if you look at the hull section (the dark brown pieces in your rendering) you’ll notice that it is, in fact, two pieces connected along the keel- This would allow the Hell to be widened. The problem with my concept was that the hull was a fixed-beam, meaning the more decks you added the more top heavy she became, consequently the more likely she would be to capsize. By making the hull wider, this danger drops. There was a Danish ship who’s name I forget that capsized on her maiden voyage because some Danish king wanted a two-deck warship; It was determined that if her hull was 2 feet wider and draft 1½ feet deeper she would not have.
Either way, a Hell of a start, Sir Pleamo; Though there is no specific “tip of the hat” smiley, I offer one just the same.
Will each section have a mast? Or will there be the option of 2 sections? 
And I see you can make it wider as well (something you have already said? 
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The idea is that each section of hull has two guns, a mast between them, and a hatch forward of that.
A “section” is length, a “deck” is stories, to clarify. You have to have at least Ø2 sections, a bow and stern. You could have Ø1 deck (though it might look a bit funny).
Wow! But how big would such a ship be? 
¿How big do you want? The idea here is to make her as big as you want- Even big enough (in theory) to make her into a catamaran for a REAL boat (as silly as that idea may be).
