Through the KC endless thread the discussion of using two came up from those still thinking in the older "two speakers = stereo mindset" and then came the
'louder" thinking. Since Aspen has said he has no immediate plans to build a smaller or bigger version in the near future it seems two stacked on a sub is the optimal setup for super loud rigs. Aspen also said "save your money" and use just one. If on a big gig with a band run a line out to the FOH system and use your SSE for an onstage monitor that the whole band can hear clearly.
It seems from the reviews that one is sufficent, especially with a sub.