Rekordbox splitting sound between speaker and headphones for laptop mixing in performance mode

I am trying to split the sound between my speaker and headphones to enable me to use the cue buttons to beat match in my laptop in performance mode (no controller). I set up an aggregate device on my mac which appears to be set up correctly and should have 4 output channels but in Rekordbox only Aggregate Device 1/2 comes up as an option there is nothing else to select so I can't see how I can select the different parts of the aggregate device as outputs. What am I missing either on the mac or Rekordbox!