At the "rotate" point on take off and the calls on descent I hear two separate voices saying the same thing, i.e. "rotate". I assume one is from the NGX & one from FSX. Is there a way to remove one of the voices,preferably the FSX one?

Yes. FMC options: PNF callouts (don't remember the exact page).

One is the airplane, other is your copilot.

Are you useiing fs2crew (?) Then that would explain hearing 2 voices.

Turn EGPWS V1 call OFF- leave Co Pilot's callouts ON