An amusing exchange from the Finale users' conference:
One example can be found in the Kalmus Rite of Spring, page 9, rehearsal number 11, in the 3rd and 4th horn parts. Here, the horns are playing D sharp and D natural together and splayed stems are used.
You can get the same effect for horns by writing unison Ds. ;-)
Puts me in mind of the old joke that goes, What do you call two guitarists trying to play the same melody? Counterpoint.
Okay, so I like geeky musician humor.
How can you tell if you're kissing a French Horn player? She keeps trying to stick her fist up your arse. Is that any better?
No, probably not.