Buy X and Y, get discount on both of them

I'm trying to find an app that gives me the option to have a BXBY get z% off *both* X & Y. (Item X and Y must be in the same cart - discount would work for each (x+y) pairing in the cart). I’m trying to make a discount where a customer buys a bike rack and a pair of panniers (bike accessories, different Shopify item categories and collections), and then receives 15% off both.