There was a third party server that would sync playback of multiple Receivers, as I recall. However, that was a PC-based product, i.e., it did not run on the Rio Central.
Since the feature you describe is not available on the Rio Central itself, and since there is currently not a full replacement software to install on the Central, my guess is that this is not possible yet.
So, to sum up:
Sync playback of Receiver+Receiver= Yes, I think so, if you use a different server.
Sunc playback of Central+Receiver= No.