This is one of most common discount offered by many online stores. This encourages the customer to purchase a second since he gets a discount.
So you end up selling two products instead of 1.
Here is the example configuration for the above discount rule
NOTE: The rule applies for purchasing ONE wedding ring and will get 20% discount on the SECOND ring. It does NOT apply the discount if the customer purchases a third. But you can always modify the rule to do that as well :-)
Notice that it is configured for Specific products. You can also make this apply to specific categories as well.
NOTE: According to the configuration, the customer can purchase any number of the above two products. However, they would only get ONE quantity of the second product with a discount of 20%
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