IMHO, more cards are not a useful solution unless one has a way to backup the other cards to the newer ones. The idea the OP has (backing up) is a good one! Having just one copy of an image file on location is scary for some of us, much like taking one camera body on location. More card with something like the UDMA then leaving the existing files on those cards represents a backup. I believe that is what the OP is asking for.
I don't believe those of us suggesting more cards are suggesting leaving the images on the card forever. I certainly do not.
My cameras have dual slots so having 2 copies an automatic function if you set up the camera that way. And depending on the realities in the field, if common mode or common cause failures are not accounted for, having multiple copies is a mute point. Flash is inherently more reliable than spinning disks and much more likely to survive impact or water trauma. Given my experience, I would rather have 1 copy on CF than 2 copies on 2 different spinning disks! 1 Copy on CF and 1 one disk would be ok, but 2 copies on 2 different flash cards would be the best. And CF much better than SD because they are inherently more durable.
The best or even a really good or necessary strategy requires more information than originally presented. Things like:
1. Is power an issue. Do I have a power source available at night or at regular intervals or am I limited to what I can carry and what is the duration of my stints between power availability.
2. Can I physically separate the 2 images and keep track of their locations or at least keep them with me, but in physically separate storage such that loss or theft of a single bag does not lose both copies.
3. How many images and of what size do I need to deal with. Big difference if you are talking 500, 1,000 or 10,000 images.
4. Do I need access to the images during my time in the field or am I just needing storage?