I have 2 soundtouch 10's that I have had paired (R+L) since that was an option. Recently, they have started 'drifting' such that one speaker is 1/4 to 1/2 second off from the other. I have to power off/on to get things back in sync.
Can you check the signal strength going to the systems using the procedure HERE? If the systems are registering "fair" or "poor" signal, that could be the cause and we should try and increase the signal strength. Also, if you're using more than 4 systems on Wi-Fi, it could be more than your router can handle. We recommend any additional speakers be connected via Ethernet.