Yes, you can. When you go to your Downloads tab at turbotax.com/myaccount, click Enter License code here at the top of the page. Choose the OS you didn't already have on your account and put the same license code in. You will then have both MAC & Windows on your Downloads tab for the same purchase.
Note that you must be logged into the same account you added your license code to the first time. But yes, it will work. That's how I've done it for years.
