Because BitBuffet doesn't have shopping carts (now we do!), a simple way to do this is to create another product for the 2-in-1 with an updated price. Then, before you add your file, zip up (using WinZip or similar) both products and upload the zip file.
In the future, BitBuffet may include a shopping cart, but until then, zipping files and creating a totally new product is the best way to accomplish this.
Just added! Use PayPal's built in shopping cart with BitBuffet file delivery by clicking the "Create Add-To-Cart" button on your product's main page near the button settings. Click here to read about creating Add to Cart buttons.