I've got a trumpet part which is divisi (upper and lower part on one staff). The upper part starts with a F, which has an accidental sharp in front of it. The 3rd note of the lower part is a F as well... Does the accidental also apply to this note?
Best practice is to put the accidental in both parts. We could argue the point in polyphonic keyboard music. But this is actually two seperate PLAYERS. Tell them both!
I'm assuming the two F notes are in the same octave. If not, the rule is clear. An accidental only affects the octave it's in. But, nonetheless, a cautionary natural where it ISN'T required would be sensible.
A "divisi" score is just a way to reduce the amount of paper used. If the same two parts were written as "Horn 1" and "Horn 2" there would be no question that the accidental only applies to the part it's written into.
Therefore, divisi parts follow only their own accidentals, just as they follow only their own slur/staccato, dynamics, etc.
The upper part starts with a F, which has an accidental sharp in front of it.
My assumption is: this is not an accidental but a key sign that says: we are in major G or minor e (trumpet setting!). If you have other parts for C-instruments with one flat in front (after the clef) or Eb-Horns with 2# (F# and C#) this answer is the right one:
The key sign applies to all parts.