Because half the power is now driving the second pair of speakers, rather than just one pair?

I don’t expect this to be the case. Connecting speakers to an amplifier is similar to connecting more lamps to a circuit in your house. You can keep adding lamps (speakers) until the circuit current limit is reached and the lamp (speaker) output will not noticeably change as more lamps (speakers) are added.

Untrained humans are very poor judges of sound intensity. For most listeners doubling the output results in a barely perceptible change for the human. After some training in a studio, a person’s ability to notice smaller changes can improve somewhat.

I didn't expect the sound to be noticable but at the same volume the sound intensity was lower when I installed two pairs speakers. One pair is Sonos by Sonance and one pair is Sonance Magv3. Is it possible that this mismatch is affecting my output intensity? Almost like using LED bulbs and incandescence bulbs in the same room to borrow from you analogy. 

Possibly the two sets of speakers are wired out of phase or placed at unfortunate locations and there are cancellations.