The backup files will be different for each PC, though they will be written to the same folder if you are using the same path to the backup location. Assuming you have an external drive that you are plugging into each PC when it is time to do a backup and are letting OneCare create the backup folder, all backups will go within the same folder. They are not overwriting each other. You can confirm this by starting the restore wizard on either PC.
An alternative is to create a Network Share and point the backup to use this Share (after you set the permissions on the Share to "everyone" having full access. You can use the same base folder or create a base folder for each backup.
-steve
Microsoft MVP Windows Live / Windows Live OneCare Forum Moderator