I don't think that you can use CiviDiscount for the work-flow you are describing.
CiviCRM does have another way of registering multiple participants, and that is with the use of price sets. You could use this to offer a discount. However, this method would only be useful fo you if you don't need to collect any information about the additional registrants.
If you went down the price set road, you would turn off register multiple participants on the event config, create a price set and and set your event fees to use this price set.
An example price set would look something like this:
- First price field: a required check-box set at $100 dollars (this handles the first registration)
- Second price field: a text/numeric quantity field set at $80 that increments the registration count by 1 (for all subsequent registrations)
Note: We are not using CiviDiscount at all in the price set example above.
Aside: "Option to apply discounts to additional participants" would probably be better worded as "Discounts can be applied to additional participants" in the documentation.