If you wish to provide two items to a customer in one sale, it is possible to set up bundles. Bundles are products that trigger the addition of two items to an order.
Bundles have the advantage over Auto-credit that they are aware of the stock levels of the items in the bundle. The bundle takes the stock level of the lowest stocked item in the bundle and so will go out of stock if any item in the bundle sells out.
Auto-credit rules allow the automatic addition of an item to customers orders when a particular product is purchased.
It should be noted that this is not a sale and the Auto-credit function should be used for example to credit the digital version of a physical product that has been sold, or adding an additional promo item in an order.
In order to set up an auto-credit on a particular Product Format go to Format Page > Actions > Set-up Auto-credit
Then choose which format you would like to auto-credit. You can also choose a format from a different product (in the case of a sticker / other promo item for example)