We did both the other week, walked out of Fantasmic, and everyone was POURING out the entrance, so much space for us to grab a good spot for the fireworks!
In the past we've done the fireworks from the crossways of the main blvd & the one leading down to Fant, and peaced quickly when they finished and still managed to get really good seats for Fant. Obviously this depends on which order the shows are in, but it's definitely do able