Listening to Audio I/F AND mixer at the same time?

[mkultra]

I have the US-20x20, how do i listen to both AudioI/F and the Mixer at the same time?

For example i'm now playing a youtube video and i want to be able to also hear what's coming into input 5-6 (tweaking a drum sound on the Nord Drum 2 to sound similar to the sound i'm hearing on youtube).

Thanks