For a project in my group, we needed to talk to multiple XBee radios from a computer. I adapted the XBee API library for Processing by Rob Faludi and Daniel Shiffman, adding methods for sending and receiving serial data (Series 1 radios only).
To send data to a specific radio (identified by its 16-bit address), you use the xbee.sendDataString16(int addr, int bytes) function. There's a new constant, xbee.SERIES1_RX16PACKET, that's returned by XBeeDataFrame.getApiID() for incoming data from a radio. XBeeDataFrame's with this API ID support the getAddress16() and getBytes() methods.