Hi Dave and welcome to the forums.
Well, the only way to fully mirror in DMX is program them that way or set the fixtures to the same address. The other lights sync up because they are in Master/Slave mode using sound as the trigger. So the slave does whatever the master tells it.
You could try setting the DMX addresses the same, but as soon as it triggers sound active mode via DMX, both lights will go out of sync. Sound active via DMX doesn't Master/Slave the fixtures like it does in stand alone. Not to say you still can't create some really great effects. One of the most useful tools I use is random strobe with a bunch of lights and add some movement. Looks more impressive and complicated then it really is.