Hmmm... I thiiink you can split your backup drive into two partitions without erasing it, but I wouldn't advise your trying without confirmation from someone more on top of it than I am. (Of course, if you're only using it as a backup and not for data storage it won't matter.)
Since OS X 10.5 (Leopard), Apple's Disk Utility has had the ability to partition or re-partition a drive non-destructively, but
only the unused portion of the drive can be in the new partition. So it is a good idea (almost imperative) to defragment the files and volume prior to repartitioning. You can find instructions
here. Prior to Leopard TechTool Pro offered the same ability and would defragment the drive as well. Prudence dictates you should always backup a drive before attempting to repartition it as accidents can happen.
