Being pedantic, 440 Hz is an A, not a C. In the case that you have given, the two flutes would sound out of tune with each other. One would be playing 335 Hz and one at 440 Hz. I admit that I misread the question, and thought that you hadwritten 435 and 440 Hz. 335 Hz would be something like an F, but still the two flutes would not be playing the absolute note.

This is what happens with transposing instruments (the flute is normally not considered to be a transposing instrument): playing C in a Bb saxophone will actually sound Bb.

Being pedantic, 440 Hz is an A, not a C. In the case that you have given, the two flutes would sound out of tune with each other. One would be playing 335 Hz and one at 440 Hz. I admit that I misread the question, and thought that you hadwritten 435 and 440 Hz. 335 Hz would be something like an F, but still the two flutes would not be playing the same absolute note.

This is what happens with transposing instruments (the flute is normally not considered to be a transposing instrument): playing C in a Bb saxophone will actually sound Bb.