Well, I don't know much about scripting for XBMC, bu tI've managed to use its broadcast function with relative success. You could maybe try making a small script that gets one XBox as host to broadcast the filename it starts playing and the other XBox to start playing the same filename when it recieves the broadcast. It wouldnt really syncronise perfectly and youd have about a seond delay between the two, but you could counteract that in the script if it bothered you.
Alternatively, just have all the music added to a playlist, launch at the same time on both and lock XBMC so it cant be changed. Not as fun a solution, and means no random songs, but it would work.