It will be slightly out of synch. I asked the same question a while back on the sonos forum and didn't get any responses. I decided to buy it anyway and tried it out myself with the following configuration: The ZP90 in the family room with the AV receiver hooked to the line-in and the ZP120 in the kitchen. I turn the AV Receiver plus TV on and then play the TV audio in the kitchen over the ZP120. There was noticeable lip synch issues when I was in the kitchen looking at the TV. I say noticeable but it's not something that would drive me crazy if I have to run in the kitchen and get something but it would be a problem if I wanted to watch TV from the kitchen while doing something.
With that said, from my unscientific testing the delay is affected by the distance between the two zoneplayers and also if there is a third zoneplayer. I say this b/c if you have the zoneplayers in the same location and both have network connections I think the delay would be even less that what I'm getting now. With your setup, will your two zoneplayers be in the same location or will they be in different rooms?
Let me know the answer to the question above and I'll rig up my system in a similar configuration and let you know the results.