BNC and USB from streamer to DAC

So, i’m sure to some this will be a stupid question but: Can i connect both USB and BNC from my streamer to my dac, so i can compare sound, and just switch sources on my dac between the two?
Usually yes, but I'm not sure how well the USB receiver will react. There's quite a bit of handshaking going on.  Try it. 

Asynchronous USB delivers just data - you cannot make it any better.  The only disadvantage  in comparison to S/Pdif is the amount of electrical noise injected into DAC, that can affect the sound.  When you have both cables connected USB should win.

The thing I'd compare to though is optical.

While USB wins for reduced jitter (normally) I'm afraid even modern DAC's still may lack galvanic isolation, and introduce ground loops, which _may_ be heard as hum, or may evidence itself as jitter (as some tests have shown).

I suggest you only have one cable connected at a time if you are comparing input types or cables.  Since a lot of the potential difference may be some form of noise transmission by the cable, having both connected at the same time wont remove that variable.  The best advice I have heard is to have only one connection at a time connected when you are making a comparison.
Thanks guys. Happy new year. Erik: my dac has galvanic isolation. Not exactly sure how it helps, but evidently it’s a good thing. Is that something that should be in all inputs, or just USB?