50% off one product when another is purchased?

Just wondered if I can get this scenario working with the existing discount functionality, or if there is a plugin that will handle it for me?

Buy a 1hour massage treatment, get another 1hour treatment at 50% off

There are 10 different 1hr treatments that this can apply to. I've setup the 'has one of these products in cart' requirement, and added all the products it can apply to. I've set the % off, and set Maximum discounted quantity to 1.

Problem: the 50% off discount applies when the customer only has one of the treatments in their cart. I need it to only apply if there are two of the selected treatments in there...

Any ideas how to get this working please?

Thanks!