Perfectly, Drashna, thanks.
I need to remove one drive from the enclosure to make room, otherwise I'd have just popped in the new drives.
But the box I'm using for my server only has 6 hard drive bays, and I'm using 5 of them. So I figured I'd remove one drive from the pool first, then take it out of the box when I put in the two new ones.
Between the "create duplicate files" and my own back up copies of files outside the pool, I think I have some seriously uneccesary redundancy going on. Having backups of files on the same drive, inside and outside of the pool, doesn't really help me when/if a drive crashes.