The best and easiest way to achieve this would be to run Tickera accompanied by WooCommerce which, essentially allows you to create and sell Tickera tickets like any other WooCommerce product. With this, you can use a plugin such as this one and make a pricing deal that matches your scenario.
Now, if you want to achieve this with Tickera itself, you can install and activate this unofficial add-on which will add another field available when creating new or editing existing ticket type. The field is named Additional Price Per Ticket and is located at the bottom. So, you can enter here 0 (zero) which will result in first ticket being sold using regular price and the next one will be automatically reduced to zero. Of course, you should then limit number of available tickets per order to 2 to avoid having customers purchasing one ticket and having unlimited number of free tickets afterwards.