You're wrong. The 0 option doesn't cost $49, it cost $0. See the screenshot:
And $0*2 is still $0 in my book.
You can't update the selection of the option after the product is in the cart.
That's why I recommended to turn on the "group options with product" option of the configuration in order to avoid any confusion between the option selection and the quantity of each option...
You need to understand what an option is.
When you buy a computer with an option for an integrated webcam, if you set the quantity of the webcam to 2 in the cart while you leave the quantity of the computer to 1 in the cart, it is not possible. That's why by default that "group options with product" option is turned on. So that the options are associated with the products directly in the cart and when you change the quantity, you change the quantity of the whole set, not of each one.