Each vdev will still have a redundancy of 2 drives. Lose one vdev and the entire pool is gone. So you can lose 2 drives guaranteed without problems. You can lose up to two more drives as long as they are in the second vdev. The redundancy does not add up.
14 is a bit wide by the way. I'd start with 2x 7-wide RAIDZ3 with 14 drives to configure. Depending on your use case, of course. If you need IOPS, don't use RAIDZ but mirrrors.