I don’t see why not, though I’m sure someone with direct experience will be along shortly to comment.
The USB ports offer storage functionality and audio output, so there’s no reason why both shouldn’t be able to function independently at the same time.
Roon recommends not using the USB output to directly connect a DAC in the interests of best sound quality and I’m sure many will jump in and excoriate you for even considering it, however, for a competently engineered DAC, it shouldn’t make a difference.
Since I posted my question I’ve changed my config. I’ll be running USB from the Nucleus to a Matrix SPDIF and from there AES to a Trinnov processor, then AES to my Topping D90 DAC. I guess my question is can I run an external SSD into one Nucleus USB port and a USB out of the other USB port to the Matrix? I’ve never used both USB ports on the Nucleus at once.