Hi Andy,
Yes, it's possible to use two masters in order to transmit more than 4 audio channels. You need to synchronize the masters in time though, and you can read about timeslot alignement in chapter 2.4.6 in the CC85xx Family User's Guide: http://www.ti.com/lit/ug/swru250l/swru250l.pdf There is also some info on timeslot alignement in the built-in help system in the PPW Configurator. Keep in mind that each slave can only be connected to one master at the time.
-Kristoffer