discount Discount for specific products when added to cart

I have a client that would like the following discount functionality, and I can't seem to work it out, seems pretty straightforward and I'm hoping it is possible on Squarespace.

When Product A and Product B are added to the same cart, they are both discounted by $x. However, if only one of those are added, or one is removed, the discount also removes. 

Context: Client is selling a tracksuit, but also sells the tops and pants separate. When buying the full tracksuit, it's cheaper than buying the two items separately. To ease the inventory numbers they want to simply discount the top and pants when added to the cart together, instead of selling the tracksuit in full as a whole other product on the site.

I've played around with product categories and discounts based on product etc. but can't seem to get it to work. My client is on the Commerce Advanced plan.

Was this feature ever added? I'm selling boxed sets of ebooks. I would like to provide progressive discounts based on how many boxed sets in the same series are added to a shopping cart.