How to configure Cubasis 3.4 with UR44-C and special routing

Hi there, I have a setting where I record with 4 mics on inputs 1 to 4 and a stereo keyboard connected to 5/6. The keyboard has no builtin speakers and only a single stereo output jack.
Now I’d like to configure a routing where all 6 channels can be recorded simultaneously, but only the signal from the inputs 5 and 6 goes to the main output for monitoring.
Does anyone know, if this is possible? Any ideas welcome