If only Horn 1 is playing, you should place a “1” above the staff where that part begins.
Similarly, if/when Horn 2 gets to play on its own, place a “2” above the staff.
When both Horn 1 and Horn 2 are playing at the same time:
- if they they are both playing the same part, write “a 2” (a due) above.
(If there was another horn sharing this staff then you’d need to be to be more specific with “1,2 a 2”.)
- if they are playing different parts, it’s probably clear that Horn 1 is the upper voice and Horn 2 is the lower voice, so you won’t need to mark the numbers. But if it’s not obvious for some reason, when the parts have:
- shared stems, write “1,2” above the staff.
- stems in opposite directions, write a “1” above the staff and a “2” below.
If the instruction is for the part to be actually played in a soloistic way, instead write “1 solo” or “1,2 soli” etc.