In the same machine?
Any significant problem with dust or smoke?
What is strange, is TWO drives going down in succession - both at the same time, and I'd suspect that some event damaged them both.
As far as I can understand, there should not be any software difference in reading, though there would be in writing.
If they are still under warranty, than do NOT even try a cleaning CD unless advised to before returning.
I'm wondering if Nero (if thats what you use) would show anything useful in the log - if it doesnt get anywhere near starting a DVD burn.
Have you updated anything, like IDE controller or chipset drivers?
Having had a DVD-ROM go incapable of reading DVDs (but still reading CDs), I fear the worst, but how the hell can two drives go the same way in such a short space of time - even if there was a faulty batch?
I'd look at any media (blank or otherwise) that you were using at the time, and see if there is some damage or contamination.