Assuming you're target is multi-room audio (only) in sync, one way to lock things up is to use an analog source. If you have the first generation ATV, you've got an analog out. If not, I like the Airport Express for this. It's a great deal, you can find them for $89, and stream audio to them from iTunes as a destination. You can simply split the analog outputs with a "Y" cord and feed both receivers if they are close by, or what I do is place multiple Airports around where I need them, then in iTunes select them as destinations, and they all play out in sync. Any new receivers that are "AirTunes" enabled will work pretty much the same. For any digital audio connection you've got whatever delays are involved in processing to analog within the receivers.