how do I create a secondary time machine back-up on the same external drive?

It sounds like you are thinking of backing up the same Mac two times, to the same external hard drive, correct?

If yes, it is much safer to have only one backup on a drive and use another drive for another backup. If one drive fails, you still have the other backup. It's your call, but if you do this and the drive fails, then you lose both of your backups at the same time.

If you really want to do this, you will need to partition the drive into two separate sections and name each partition differently. Then set up Time Machine to back up to each partition. But, the same problem as above remains......if the drive fails, you lose both partitions and both backups.

Sorry, but you will need to add another hard drive. It's too late if you already have data on the drive now......since you cannot partition a drive unless it is blank.

Buy a USB drive and set up the second backup to go to that drive.