Never tried using both at once.
(I guess you would if you used your computer with another operating system in which the drivers don't support muting/unmuting digital cd audio from your sound card)
If both were working(believe it or not, some CD-ROM drives are manufactured with both connectors, but the digital isn't actually enabled) then I would simply use the digial output for cleaner sound.
To your burner from your CD-ROM? Definitely doesn't do anything.
From your burner to your soundcard? Sure, then you can get cd audio from your burner
