First, we are talking about SPL. SPL is not loudness. Loudness is sensory intensity, how loud it SOUNDS. If you do frequency shaping, loudness and intensity do not track.
But if two signals are direct inverses they will cancel in a perfect sum. If they are identical they will add to 2x the intensity. If they are orthogonal, they will add to 1.414 times the intensity at the position they are orthogonal. So, even understanding the sum requires knowing the transfer function from each speaker to the microphone (and is that a pressure sum, a 4-dimensional sum (including volume velocities, or what).
But please use "loudness" only when you mean the sensory intensity. Intensity by itself is a cognate of SPL (although depending on how you measure the intensity SPL may be missing some components).
http://www.aes-media.org/sections/pnw/pnwrecaps/2014/jj_jan2014/ is a partial discussion on loudness vs. intensity. The later hearing tutorial will also explain some aspects of loudness.