A digital out only carries a single PCM (mono) signal so I don't think that's going to happen.
It also operates at a different voltage than analog outs, so there's no way it would be safe.
Finally, qualitatively, SNR levels should not be a major difference between your onboard and your PCI soundcard. The difference would exist in possible EMI (interferrence). Usually this can be cancelled out thru muting all the non-playback line outs.