Where identical really means down to last byte so it should better be exactly same manufacturer/brand, not just two random 8 gig cards. Output of cat /proc/partitions tells exact size.

if your second card is larger then you can copy to file (so you can have other files there too)
or you can pipe it through gzip to make it smaller (may take longer)

This is all easy but also very wasteful since you are backing up unused space too. More sophisticated way is to backup partition table (via dd or sfdisk) and then backup just files from each partition (via tar) and later restore partition table, re-create filesystems and then restore all files.

Best would be to port partimage or similar tool to maemo.