If it's a bootable RAID 1 array, then it's using both drives, however it should be able to run off a single drive in degraded mode unless of course a virus messed up the files on those drives or you accidently deleted a needed system file. Remember, in RAID 1 whatever gets written to one drive gets writen to both. RAID is not a backup solution. It's only meant for redundancy in order to keep a system running if a drive fails.
Typically whichever drive responds that it had the file ready to transfer first is the one that will be used for that one file. Thats how raid 1 is supposed to work but I have seen s few instances of 1 drive always being the primary drive and the other a dedicated mirror that never gets read. I've also seen a few cases where the drives altrernate which one gets to be the primary drive.