As you probably know, two drives actually "failing" at the same time is unlikely. If two drives go offline at the same time, there could be a number of other reasons why:
Backplane - multiple slots - mostly on same channels (like all of disks 0-3 or 4-7 ... so a RAID 5 with disks 2,3,4,5 and channel 1 fails, 4,5 would go offline)
Cable - some 2600's shipped with cables that were prone to errors; damaged cables can do this too
Controller failure or glitch - failure to communicate in a timely manner or an inability to recover from certain types of errors (many of which can be avoided by keeping controller firmware up to date) can cause random drives (or particular ones) to go offline; events may also cause the RAID config on the drives or controller to become corrupt, causing it to have a different signature - and if they do not match, they are marked as "foreign" by the controller and not used in a RAID array - if two disks are marked as foreign in a RAID 5, then obviously, the RAID 5 would not be put online
Corrupt RAID array - if part of a RAID config is damaged and the controller can no longer reconcile the data/parity, it is very common for a disk to go offline (or fail a rebuild, Consistency Check, etc.); if multiple disks have similar issues, then multiples may be taken offline (this is usually a catastrophic failure of the array).
What system is yours? What controller? What RAID config? What happened?