I can partially help you. Here's what I do to back up
my SDcard. I think it should also work fine to back up the internal flash:
First, on the Backup Machine:
$ nc -lp 2345 | gzip >sheeva-root-fs-$(date +%F).cgz
Then, on the SheevaPlug:
# BKUPIP=<IP address of backup machine>
# cd / && find . dev -mount | cpio -oc | nc $BKUPIP 2345
You can interrupt the processes on the SheevaPlug after the "XXXX blocks" message appears there, and you should then have a good cpio archive on the backup machine.
This should create a remote backup of your root file system (plus /dev, some of which is necessary to boot). However, I've never looked into what is necessary to get it onto a second Plug's internal flash memory because I don't generally use this configuration. I suppose if it were me, I'd boot the second Plug off of an SDcard, set up the internal flash as a mounted FS and restore it, and then reset the Uboot parms to boot using the internal flash, but perhaps someone else can suggest a more elegant solution...