Quantity Discounts For Collections

I want to offer quantity discounts for collections. Is that possible to set up? This is the scenario:

I have a 'download all photos' package for $195.
If the parents have two children, I want them to buy two of those packages. So I put the two children in separate galleries so that they can't pay once and get photos of both their kids.
But, they demand a discount.
So I created a 'sibling download all photos' package that contains 2 (No.) download all photos.
But on the user's end, they can only add one child's gallery to it.
How do I solve this problem?
Is there a simple way of just creating quantity discounts so that if someone has two of the $195 'download all photos' package in their cart, it automatically creates a $15 discount?

Edited by

There is just not a way to do that. Only thing I can think of is offering those customers a coupon for the discount.

Edited by