Creating a product with one or two required grouped items

I am trying to create a product where the user selects either one or two different t-shirts (They can choose black/white for the color and then the size for that color) that get added to the product (package) they selected. The t-shirts are required part of the package though. It would be like this:

The package is a box with a book, a thank you card, and then depending on the package they select, they get to choose either 1 or 2 t-shirts that are included as part of the package. I want to be able to manage the inventory of the t-shirts across multiple products, but the t-shirts must be an included item because the package comes with a t-shirt (or two t-shirts depending on the package they select). Right now I am setting it up as a variable product and using the variations to set the color and size of the t-shirt, but with those variations, I cannot manage the stock across multiple products, only on the one product but the t-shirts for each product will be the same.

What’s the best way to make this happen?