It depends!
Early August usually has two showings of Fantasmic, and mid-to-late August will go with one show. Usually the turnover takes place around when school goes in session in Florida.
This summer, we were there on the August 17th when there were two shows, and by the 19th there was only one show.
If you go when there is only one show, it's impossible to do both Fantasmic and Illuminations. When Fantasmic is at 8:30, you won't be able to watch and travel to make the 9:00 Illuminations. If there's two shows, the late show has the same problem by starting too soon after Illuminations is over. This is considering summer hours -- it's different in the winter months.