How to apply discount to only 1 of 2 (same) items being exchanged

I have a question about the Shopify POS app.

I have created a new order using the Shopify POS app. I would like to exchange one of the line items for 2 different line items (i.e. same new item but with quantity of 2) - but I would like to do it in such a way, that I can apply a discount to only one of the 2 items I’m adding.

For example, I’d like to exchange 1 laptop for 2 monitors (which are the same product), but in such a way that only one of the 2 monitors is discounted at 20%.

I struggle to find a way to do it in the Shopify app - it only allows me to discount both monitors.

Thank you,

Piotr