Apple - How to synchronize contents of two external drives?

You could use rsync like this:

rsync -av --delete /Volumes/DiskA/ /Volumes/DiskB

--delete option is for delete files in DiskB that was previously deleted on DiskA.

If you want to schedule it to execute periodically, I suggest you to read How can I run/stop/relaunch an application automatically, at boot/login/some other time?.