Multiple Bluetooth Speakers

Audio MIDI Setup to the rescue!

Open Audio MIDI Setup which is located in Applications > Utilities.

Click the + icon in the bottom-left hand side and choose Create Multi-Output Device.

When the multi-output device is selected, click the Built-In Output, and the Cinema Display's output audio devices.

After this is done, open System Preferences and click the Sound category. Under the Output tab, choose the Multi-Output device.

Any applications that were open when you started this process may have to be restarted in order to pick up the change in default sound devices. Some do, some don't.

I do believe that should do it.

According to Wikipedia:

The AirTunes part of the AirPlay protocol stack uses UDP for streaming audio and is based on the RTSP network control protocol.[12] The streams are encrypted with AES, requiring the receiver to have access to the appropriate private key to decrypt the streams.[13] The AirPort Express' streaming media capabilities use Apple's Remote Audio Output Protocol (RAOP), a proprietary variant of RTSP/RTP. Using WDS-bridging,[14] the AirPort Express can allow AirPlay functionality (as well as Internet access, file and print sharing, etc.) across a larger distance in a mixed environment of wired and up to 10 wireless clients.

See the page here.

Notice the part where it says that UDP is used for streaming audio. UDP does no error correction. When using TCP (most of the time), when I send a packet from A to B, I send it, I get confirmation, then I send the next packet. When I get confirmation, part of that confirmation is an error check code. If it doesn't match with the error check code that I generated when I first sent the packet, I know the data has been malformed in some way before reaching B, and therefore I send it again.

If I have to send that packet a bunch of times before it is successfully read on the other side, that would "sound like" something unexpected. Lag is a bit different -- lag happens when there's a delay of some sort, or the line is saturated and it actually takes (some arbitrary amount of) time for the signal to get there. That could happen under either model.

The difference is, with UDP, if packets get lost or corrupt, it doesn't matter. iTunes will send whatever data is required to represent the song now. And now, and now. If you miss it or it gets malformed, oh well. It will keep sending only what is pertinent now.

Long story short: you should be fine. Any glitches will probably be short and sweet. And, out of sync? Probably not discernible to any human.