Summed Stereo

[Shannon]

Two separate speakers producing separate audio outputs from different physical locations will always produce a better result than a single speaker projecting two different channels that are summed, as you call it, because there is still the valid spatial audio. Summing does not give spatial audio, so you are in effect removing the spatial audio from the playback which means it won't work as well.

Summed is, however, a better solution than pure mono with only one channel being played back.