When you beep() it causes a detectable spike on the mic input, this means you could:
-Connect the mic inputs of both cameras together
-Have the "master" camera beep every time a picture is taken (you will probably have to modify ML to do this, but it should be an extremely easy modification).
-Use the "audio trigger" in the "slave" camera.
If you're in a quite environment then you wouldn't even need to connect the mic cable, just have one camera beep() and the other camera with the audio trigger.