So many of those USB equipped mixers only have a stereo out for it’s USB Audio, it’s frustrating as so many people get wrongly fooled into thinking you can record all 8 chans independently.
And raino above is right - the only way is to Pan Hard left on Mic 1’s channel on the mixer and use the Left input (mono) in Cubase track 1, and hard right on the other and set Right input as source in Cubase for Track 2.
If you’re not doing a live recording you may be best to record each Mic seperately too so you don’t get bleed, each singer can monitor via headphones too. But depends on the application, of course.