Doug, the general increase for two widely separated speakers fed the same signal at the same power level is 3dB. When the speakers aren't widely separated but are instead "co-located" an additional increase of up to 3dB can be had at certain frequencies. This is more often discussed with respect to subs, for example in this
Electrovoice brochure at p.3 under "use in multiples", but the same basic principle applies to all speakers. When their drivers are located less than about one-quarter wavelength from each other they "couple" and the increased pressure that their cones cause on each other at certain frequencies results in higher output. For example, if the center of the drivers is about 1' apart, that corresponds to a wavelength of 1130Hz(sound traveling at about 1130 ft./sec)and one-quarter of that would be about 280Hz, so as much as an additional 3dB increase, for a total increase of 6dB, would be gained under about 280Hz.
