"Joining" unit offerings allows you to write a single unit guide and use it for multiple offerings. Students can still search for and look at either offering, but the information that is shown will be exactly the same (except for the offering code). The best time to join offerings is before you've started doing any work on their unit guides. Offerings can only be merged if they are have the same unit code and are in the same academic year.
If you want have offerings that are almost the same but not identical, you should use cloning instead. Cloning allows you to copy the content from an existing unit guide into another offering.