If you try to play audio in iTunes to two different AirPort Express base stations, and the base stations are on different networks, you may see this alert message:
"The remote speaker is currently being used by someone else. Please select another speaker from the menu at the bottom of the iTunes window."
This issue affects iTunes 6.0.2 and AirPort Express, and it happens when the AirPort Express base stations have the same IP address on different networks.
The key to avoiding this issue is to set up a valid network controlled by only one router. If you have one AirPort Extreme acting as the primary router, for example, it would not assign the same IP address to two other devices in the network. However, this may happen if you have multiple routers active.
To achieve a single network, AirPort Express units should join the network in client mode, WDS mode, or be used as part of a roaming network (in which multiple base stations connect to a central Ethernet "backbone" that is routed by the primary router, which could be an AirPort Extreme, for example). For help with more complex setups like roaming, please see Apple Manuals.
This document will be updated as more information becomes available.