Thank you for your question regarding the stereo pairing function of the SoundTouch 10 that is not featured with the SoundTouch 20 and 30 speakers.
It's true that the SoundTouch 20 and 30 speakers cost more than the SoundTouch 10 speaker. The reason being is of the acoustic design and number of drivers used for each system. The SoundTouch 10 only comes with one driver while the SoundTouch 20 and 30 has 2 or more. Stereo sound works with two channels, left and right speaker. This being said, the expensive speakers already produce stereo sound. Adding the stereo pairing feature in these speakers will not give you the ability to separate sound in 2 channels unlike the SoundTouch 10 that you can place in 2 different locations separating the sound created in each channel.