Personally, I would use this method over the second one, because if this happens:First option: Continue using RAID1 as currently set up. This results in 2 volumes of 2 drives each. Volume 1 is Engineering Business sync/archive, while volume 2 is Personal sync/archive, and I'd probably point full-PC backups to Volume 2 as well.
In that case, you will still have access to your data and be able to rebuild with no problems. Better then to have a single drive as a single volume. For that scenario, you will definitely need a separate backup device. Also having a backup destination for your data on the same device (enclosure) is not really a backup, is it?Will one allow for easier drive replacement than the other if a drive does fail?