You are on the right track, if when either monitors are hooked up to the r/s it works, and when hooked to the l/s it does not, then I think it is safe to say that the l/s is bad. Have you tried removing the card and the drivers
, and then reseating th card, then reinstalling the drivers? But wait, you said when you go into the display settings, you see both monitors when they are both hooked to l/s and r/s? Have you tried clicking the detect button to see what happens? Sorry I have more questions than answers.