DIM4 will rename files as it downloads from a card reader. It has a naming option of '%O', which will pick up the "OwnerName" from the exif data. I just tried it with a 20D file.
You could name one 5D-1 and the other 5D-2, or whatever works best for you.
DIM4 is free at
http://www.alanlight.com/dim/Dim.htm
John