Rather than do it manually with 2 separate volumes, why not just have an RAID1 volume over the whole 8 drives? (Assuming you meant 2 volumes of 4x3TB drives for each volume)
That said, I'd just run RAID6 or SHR with 2 disk redundancy (essentially the same thing) across the one volume. The chances of 3 disks failing at once are pretty slim and you get the most bang for your storage buck that way. You seem to have a solid external backup solution in place for the critical things so the masses of redundancy you'd get from RAID1 or 2x RAID0 volumes seems like wasted space to me.
The only real issue with RAID6 is that it does take a long time to rebuild a degraded array when using such large drives, but as you can still use the system while it's building that isn't even such an issue.
Anyway, there's my 2 cents.