"RAID 6 stripes data on a block level across arrays just as RAID 5 does. But RAID 6 uses a second set of parity codes for each data stripe. Because RAID 5 uses only one set of parity codes, it can only reconstruct data on one spare disk. If another disk fails at the same time, data is permanently lost. If a second disk fails with RAID 6, data can be reconstructed onto a second drive."
via Hack the Planet