The dinner package seating is in the central area of the theater (usually Mickey, Scar, and sometimes Sebastian sections) for the first show and then FP seating is all the seating house left. If you arrive toward the end of the FP window, you'll probably be sitting over in the far left of the theater. If you arrive toward the end of the dinner package window, you'll still be in a middle section.
If there are two shows, there are usually no dinner package for the second show, so FP seating becomes the entire middle and left of the theater (so what was dinner pack and FP in the first show). Granted, this could change on the whim at any date, since it hasn't been 100% consistent since FP for Fantasmic started happening.
So basically, first show FP for Fantasmic! does not reserve good seats, it just reserves a seat. Dinner package is best seats for the first show. I recommend the second show because FP seats are better seats. Lately, it seems the second show has been after DHS has closed as well so you can probably go on attractions with minimal wait while the first show is happening and then head over to Fantasmic once the park closes.