Keep in mind that it takes a while to get out of the Fantasmic theater unless your sitting right next to the exit and you also have to consider you need to make your way over to the viewing area for the show so even though you'll make it there in plenty of time, you probably won't have the best spot. If at all possible, go on two separate nights and see one show each night rather than both in one night. I've done this for the Jingle bell jingle bam fireworks and most of the areas were packed full by the time I got out of Fantasmic.