Yes, this company charges a lot of money for not well tested (and/or developed products)
You are doing nothing wrong: Two USB connections at the same time do not work reliably. I have tested it all through, I work in the IT industry and my job is to find errors, so I am pretty sure, this IS a problem of the mixer…
See my post here: Denon X1850 two USB connections at the same time do not work reliably