People have reported they were able to do both, but it means you have bad viewing for at least one of them depending on which one is first. If F! is the second show, you do run the risk of not getting in at all if it's a busy F! night.
This!
On X night, some guests who tried to see both, got to see both. But, will that happen for you?
It absolutely depends on so many unpredictable factors that it cannot be something to "expect."
There are many nights when the Fantasmic! theater will be full by 10-20 min. before start-time.
Or, at least have a line of late-arriving guests so long that the show will start
before those waiting can find a spot.
Anyone who wants to try this combo of shows may have a fighting chance, but do not
"expect" it to happen, nor "relax" with the idea that it will be likely on the night(s)
you happen to be there.
Why does this matter?
I would rather a guest be "pleasantly surprised" and happy about being able to see both shows, than to be "shocked and disappointed" with WDW that the
expected two-show viewing didn't work out.