reasonable use of RAID 1?
 

If I set up a 2-disk hard or soft external RAID 1 solution, the normal approach seems to be using one drive as the main "working" drive and the second as the backup.

What about using both RAID drives as dual backups for a 3rd, internal drive, and having any new or edited files saved to all three at once?

that is a bit too much disks for just security. if you need an excellent security level, go to raid 5 or 10

Remember that any RAID set in a redundancy mode (RAID1, 5, 10) will be slower than RAID 0 during captures. RAID1 sends identical data to two drives at once which might be fine for SD and HDV but can lead to problems with ProRes and higher codecs during capture. RAID5 will time out for a moment to send parity data to the backup drive. This can disrupt any real-time capture with dropped frames, I speak from experience. You might want to set up a system of capturing to one set of drives and transferring to the RAID before editing.