Noteheads And Voicing In The Orchestra

I am working on an orchestral piece. Two bassoons are playing. On one staff, do I have to write the bassoons as separate voices or can I write them as one voice (like piano writing for an octave).

I've seen it done both ways. Picking up the closest five scores to me:

• Dover edition of the Marriage of Figaro uses single stems for two (of two) bassoons playing in thirds in bar 292 of the Finale of the second act
  
• Barenreiter edition of the same bar writes it in two voices
  
• Eulenberg edition of Berlioz's Symphonie Fantastique uses single stems in bar 83 for that (for two of four in this case, though)
  
• Eulenberg edition of Mozart's 23rd piano concerto uses single stems in bar 387 of the third movement in bassoon only, for ; clarinets and horns (even though they're also playing in that context - i.e., independent voices with the same rhythm) have it written in two voices
  
• Novello edition of Elgar's cello concerto has the 73rd bar of the second movement written in single stems for parallel thirds and oblique motion into a fifth
  

I think the cleanest and clearest way is to write it with a single stem if the two instruments move consistently in parallel motion (especially 8ves), and writing separate stems if they are much more separated in terms of direction of motion or the interval by which they move in similar motion. I don't really know, though; I only answered because no-one else did; maybe I'm missing something contextual in each of the scores that contributes to the way it's written and someone who actually knows stuff will post with a definite answer and make me look like a jackass.

What I've seen so far is that a single stem is used when the note values to be played by each instrumentist are the same, no matter the intervals. If the paired bassoons are to play different note values, it's better to use independent stems.

only if rhythm is different that will force you to use 2 voices, if not 1 is enough, if they are playing the same note use "a 2" or "I, II a 2" something like that.

I write separate parts. That is standard practice now.

Not if you only compose study scores :D

I find scores easier to read with less staves, and then I don't care so much about stemming

if values are the same single stem is correct,

if both play the same note then write "a 2" or "I,II a 2" something like that,

If values are different in one measure then next are the same and then other segment different etc, write two voices always and don't use "a 2"

Just follow the standard rules for voices. Same rhythm = same stems. Different rhythm = different stems. These aren't Bach fugues or four-part chorales.