USB and Coaxial at the same time?

If you have native dsd turned off in the aurender advanced settings I think you should be fine. What makes you think the coax could sound better just curious…

No but it would be weird and not a direct comparison. As soon as you connect the USB it may tru to take over the streamers clock.

A better comparison will be to do each separately

When I compared USB to coax on my N200 I was running one interface at a time and was taking it slow evaluating sonic quantities of each listening to complete songs or even more than one song with each.
I don’t recommend switching back and forth to quickly A/B. That is a waste of time and effort - you won’t have enough to go on. 

Well to be clear, I highly prefer USB. But I was wondering if on some of the older Rock recordings if switching to coaxial may smooth it out a bit. I just wanted to try it and see what happens.

It might smooth it out.