Our family loves to watch Fantasmic, however, we personally would not leave MK just to go see it. Not that it can't be done, but because we are not PH people therefore, we like to stay in one park each day. Also, the distance between MK and DHS is much farther out. If you were really wanting to do your plan, then I agree with the PP that suggested seeing it on your Epcot day because that park is much closer (you could even walk there if you were up to it), but then on the other hand, if you are wanting to see Illuminations (I like this show much better), then that could be a problem. Good luck deciding.