The current promotion system is weak in that regard. What I did on my system was create an additional product.
In promotions, I created a coupon code for the second product, where it required the first product.
The other option would be to create a Combo plan with the discount built into the price.
However, The reason I went with the method I described was to allow the client to see the discount in the cart.
For some reason the configurable addon discount option does not work or at least I can not seem to get it to work to where it only discounts the configurable addon.
Either I am missing something or it does not work perfectly.