If you are using the Squeezebox Server, you can use any Squeezebox app to synchronize different streams. (Squeezebox has a protocol to make all Squeezelite players DLNA compatible, but not synchronizable through this protocol)
If you are using Shairport AirPlay, you can use iTunes to synchronize streams.
You can check the status of your players in the Audioplayer menu of each device/zone in any browser of your choice.