I'm not sure how it works when you already have a ticket for that day, but, in every year past, I have bought a Halloween Party ticket and have been allowed to put 2 Fast Passes on. One for 4 pm and one for 5 pm. Its bcuz theorhetically you can enter into the park at 4 pm, but MK shuts down at 6, so many people get 2 FP's. That's assuming you don't already have fast passes for that day already on a "normal day ticket".