Typically they only do a second showing of Fantasmic at very busy times. I would be VERY surprised if there were second showings during the time you are there. We go two to three times a year (typically April/May, October) and have never had the option of second shows during those times. The only times we've experienced them were during Xmas-NYE week, week before July 4th (two of the busiest times of the year) and early August.
When there is one show only, it typically coincides with closing time of the park, which is also long enough after sunset for it to be dark. Check sunset times on weather.com and that should give you some idea... round to the closest half hour.