Do you think 6" apart sounds a little tight for pepper plants? Well, maybe it's a little tight for big bells, but not by much. Standard recommendations are for 18" to 24" between peppers, but research on cayenne peppers in South Carolina suggests that much closer spacings yield about one third more peppers per square foot than a 2' spacing. In the two-year study, highest yields resulted from 6" spacings in single rows. In double rows, the 6" and 12" spacings yielded the same. Yields per single plant were smaller than at 24", of course, but the total yield was much greater.
The downside of tight spacing is that it takes more plants to fill the space and you get smaller fruit. Don't use the 6" to 12" spacing when you want big, blocky bells for stuffing, or for any variety that wants to become a large bush. But for the small-fruited peppers -- cayennes, cherries or long, slender fryers -- pack them in and you'll pick more peppers.