I would just stick with telling the older one and deal with the fallout. Although you could wait until closer to Christmas. You can always solve the problem of the little one later. She's going to find out eventually, if she doesn't really know now, so it's going to be a moot point.
If your DD is not asking specifically if Santa exists and already has her own beliefs justified, is it really best to ruin the magic of Christmas at the age of 9? When she is ready to know the truth, she will ask the questions. If she still believes wholeheartedly in Santa, I say squeeze one more year of Christmas magic out of her and enjoy the heck out of it!
In our house, Santa stops coming to you when you stop believing in him. Then he lets your parents know and they start buying you stuff (and, of course, the stuff is not nearly as good). I remember still wondering when I was a teenager. I LOVE that no one actually came out and told me that the truth. I would have been devastated to hear it at that age.
This is one of those sticky parenting topics that has no right or wrong answer. You know your child best. What would be more upsetting to her at the age of 9... ribbing from some aquaintances about something she strongly believes or being told the truth when she doesn't want to hear it?
