How to give discount on the subtotal of selected products only?

I want to set up a promotion for a client in this way that if the customer buys two products from a list of selected products, then they get 20% off on the subtotal of these two products only, and not on the entire order as the customer might be having other products in the cart too and I dont want the discount to be on the total of all the products.

Can this be done in cs cart multi vendor?