Thanks for the post, and welcome to the community!
The only SoundTouch system that's able to create a stereo pair is the SoundTouch 10 system. If you do decide to get a 2nd SoundTouch 20 system, you can do "Play All" to multiple speakers to sync audio (however it wouldn't be a stereo pair since both speakers are playing both L+R audio).
With the new generation SoundTouch speakers, you can connect a device via bluetooth to play audio, and play all to multiple speakers. This works with all SoundTouch speakers.