If you don't have the second, you can purchase one of these - and will need to take it to the dealer to have programmed. But even if you want to buy another original key set from the dealer - they will still have to program the keys.
But with dwindling stock - may be a good idea to have 2 factory style keys.
To program yourself you have to have 2 main keys already programmed to add a 3rd, 4th etc...
You can't add a 2nd key on your own, as that wouldn't be too safe, (ie. a valet gets a key cut, and copies the one original - the odds of having both originals together at the same time is harder..)
I guess Ford Thought more about this part, and gave an afterthought to the key design itself!