Step 1: Create the benefit
In Marketing -> Benefits, double-click Add New Benefit. Give it a name, then click Add Condition. From the dropdown select Ticket.

In the example below, two conditions are added such that if a customer buys tickets for two different shows, both tickets are reduced to £6 each.

Step 2: Create the Discount Scheme
Under Marketing -> Discount Schemes, double-click Add New Discount Scheme. Give it a name, and click Add to add the benefit created in Step 1. You can also restrict the discount to the transaction type, and day that it will be active.

To make the discount apply automatically, be sure to tick the “Apply Automatically” checkbox.