When I look into the manual’s Signal Flow diagram it seems that MIX 1 is going out to outputs 1 L/R(+phones1) and MIX 2 is going out to mix 2 L/R(+phones2).
But in Cubase it could be possible that you can feed the 4 outputs with 4 signals coming from the DAW, that I can’t tell, you gave to see that in the outputsection of your Devices setup in Cubase.
I must say this is very restricted in compare t o my UR816C, which has an extended output-routing.