Good for you. You didn't fling both of them at her, demand compensation for HER mistake, and threaten to call corporate (I know, there isn't one) and have her FIRED!!!!!!!!!
Way to go :-D
Gah, I hope if I ever DO react that way to a simple mistake, no matter WHO made it, that any other kindly customers in earshot take me aside and kick my idiot ass all the way to the street.
My biggest actual concern when she made the TWO drinks instead of one was that I honestly thought I'd only paid for one, and if it had come down to it, I'd have paid her for the other one too. It would have messed me up on cash on hand later in the week, but as I said, those drinks are tasty.