This mod permits you to offer this type of discounted cross selling in your OpenCart.
You may specify
* Buy item <x>, get item <y> at a discount
* Buy item <x>, get an item from category <c> at a discount
* Buy an item from category <a>, get an item from category <c> at a discount
* Buy an item from category <a>, get item <x> at a discount
Discounts may be specified as percentages of the latter item's price or as
absolute values in the currency you cart uses.
With the basic module, you must edit code to add new discounts, but there is also a Better Together Admin module available in the OpenCart Marketplace. Better Together Admin provides a web based admin panel for adding and deleting discounts.
If you want to see the process of creating a Better Together discount without the Admin panel, here's a video that shows this: