Depends on your usage, some do it, some don't. You could use the second partition for other data (which will not get backed up of course) or even a bootable partition with a clone of your 500 GB HDD (one partition for Time Machine, one partition for the clone).
For my own computer I do not partition, I keep separate Time Machine backups on separate physical disks. But for family and some customers who do not care about historical backups (only the current backup), I partition one disk and alternate Time Machine between them.
Because Time Machine destinations eventually corrupt themselves. And once they do you're better off reformatting the volume and starting over. If you have just one disk for Time Machine, and it's big enough to partition into two volumes, you can have two independent Time Machine backups, and periodically blow them away. One strategy is to keep one of them enduring historically for as long as it will allow until it craps out with errors - and the second one keep relatively fresh, i.e. reformat the volume every 1-3 months. Another is to alternate formatting the volumes, which means you never have a particularly long history of backups - maybe just a few months worth.