Ryan, stacking steel, and the impact of the gap, is unknown/untested. My concern is that the areas where the steel touches will transfer considerably more heat than where it doesn't touch, and the top steel might not be able to transfer that heat evenly, giving you hot spots. But that's just a theory.
I've theorized about a few possible materials to use as a thermal compound between the two steels, but that, again, is just theory.
What's not theory, though, is the importance of pizza size with New Haven pizza. A big part of what makes Polishpizza's pizza so great is their size- 17.5". 14 x 16 is way too small for NH (and NY, for that matter). So, since you'll be needing a larger steel anyway, you might as well get a 18 x 18 x .5 one (or as big as your oven will allow).