instinct no headphones with interface

i cant seem to get the headphones to work at the same time as my speakers. 

my speakers are connected through an audio interface and so i am using the rca outputs on the controller.

on the audio set up in djuced when i click on the interface it only has the option for master 1 + 2 the headphones option is on the same scroll down and again the only option is 1 + 2 so both can t be used at once.

i ve tried calibrating  extra channels in the macs midi setup and then selecting them through the instinct aggregated menu but to no avail.

any help would be greatly appreciated.

🙁