I was wondering the same thing. But the dual audio out has some pretty serious limitations. Trying to remember what they are...
Oh, it's right on the setting's help text. Assuming that your primary output would be the HDMI, the optical would only get 2-channel, 48 kHz PCM. So no, that doesn't allow the D-BOX to work while still being able to use the lossless audio over HDMI. The only way D-BOX works with the PS3 is to set the primary output to Optical, with Bitstream. Which results in the lossy core audio. So it's compatible, as they say, but at a loss.
