A single stereo ceiling speaker (also called a dual ceiling speaker) provides full stereo sound from one single speaker unit, instead of 2 more mono ceiling speakers. Single stereo speakers are best used in smaller rooms such as bathrooms.
The most popular ceiling speaker set up is with 2 speakers (pair), one which receives the left channel of audio and one which receives the right channel of audio. This then results in full stereo sound when both speakers are played. A single stereo speaker is able to receive both the left and right channels of audio and play full stereo sound from one speaker.
A dual ceiling speaker is able to product stereo sound because it has 2 tweeters, whilst a typical speaker only has 1 tweeter. This means each tweeter will have its own channel of audio (left or right).