Help setting up 2 separate mix busses


I am trying to set up two separate output mix busses in Cubase Essential…I want one separate stereo output for drums and bass, and a second stereo mix for everything else. My audio interface is a Motu Microbook, which has one main out (stereo) and one digital Line out (stereo). The problem is I cant separate these two outs from each other, if you know what I mean…I want the drums and bass on the digital outs on separate monitors…
Appreciate any help

The Motu Microbooks outputs cannot be separated, the digital out is just a “duplicate” of the main outs.