You bought two drives, each is covered, but they will only replace a drive that is confirmed bad. As was mentioned if you have a drive error it should be very easy to determine which one went bad with simple testing - likely with WDs own test software.
Just because there is a data error or data corruption or a lost RAID set does not mean that one of the drives is bad BTW.
This is actually a good question since in the event that they don't have your exact drive in stock, they could possibly send you a slightly different or better model which would screw up your raid 0 if they sent you a bigger drive. A while back ago, one of my drives kept getting an error that would show every so often when I booted up. Even after formatting the drives and setting the raid back up, I was still getting the error. Some windows error about one of the raid disks having problems and that I should backup or it could fail. Since I wasn't sure which drive it was, I just requested an RMA for both drives. This also protected me in the event that they sent me a different model.
Another good thing about WD is that they are one of the few companies out there that offer cross shipping RMAs as long as you have credit card info. So even if one of the drives that you send back isn't bad, they don't care since it wouldn't be worth the hassle to try to trade it back for your replacement drive.
One bad thing about WD is they have a past history of failure beyound makers like Seagate.
I could care less.
In November I bought a bunch of Seagate 7200.11's 500Gb HD'sfor $89 (the best deal in my mind)...in December I bought a bunch of WD 500GB HD's for $89 and just this week I bought a bunch of Maxtor 500GB HD's for $89....all RETAIL and all in USD's.
I just build out systems and I don't care...other than keeping 7 of the Seagates for my home LAN as they rape Raptors.