As far as I know you can't do that. I ran into the same issue with trying to simultaneously output front L/R via analog and rear L/R + center via optical from the sound card. You need an external processing unit to accomplish this, like an Emotiva UMC-1.
My Sondigo lets me do this, but only in stereo. Any other CMI8788 based sound card is going to be the same. It allows both the Optical out (PCM stereo) and Front speakers output to be selected simultaneously in the control panel.