AFAIK, the UG window is determined by the lowest status level if a pax, in one/single reservation, i.e. PNR. So, in your case, the GM window.

If, the only reason you want to book it as a single rez, is because of the C+ seats, one way to do it is: book it as TWO separate rez. (two PNR's, one for each pax) and call the PM line, have them tie the PNR's together, and that way, the PM agent on the phone can assign a C+ seat for you and your companion. Also, this allows you to be UG'ed at the PM window, and your companion, at the GM window!

Safe travels.