Correct, book the 2 separate reservations, and as eticket said, if its available in a different room type for the night you are replacing book that as well and set up the waitlist to replace that 1 night if it comes through.
If you want the friend to be the one notified then you would put her in, however if you want to be the one notified then you need to put yourself in as the lead guest. You can have member services change that later on.
If the night is available and the waitlist does not come through, which does happen quite often, book it yourself.