In RAID5, that circumstance means you lose all your data
With unRaid, if you lose 2 drives at the same time, you will only have lost the data that is stored on those particular drives, and if one of the disks was you parity disk, then you will only have lost the information on one of the data drives.
Note that you can also run unRaid with dual parity drives so that you can lose up to 2 drives at the same time with no loss of data.