Dallas Weekley and Nancy Arganbright, the noted team of duet players and editors, have decided it is better to print the Primo and Secondo parts on the same page, one on top of another, with all the measures aligned. All of their recent editions of duet music do that.
My reason for not going that way and instead opting for the left-right page is that:
a. It could be mixed as a piano duet in two pianos, quite easily (and frankly this is what traditionally looks like)
b. It's far harder for each pianist to actually follow while performing. (more about that later)
However, I've also seen recent editions of duets that print Primo on the right and Secondo on the left. These recent editions have made improvements from horrible, older editions in these ways:
1) the editors have clearly labeled measure numbers instead of the dumb rehearsal "letters,"
2) each system (between Primo and Secondo) starts on the same measure in the music,
3) the print is large and clear, and
4) there is a lot of margin and space between systems in which to pencil in rehearsal notes.
Without having studied other quatre mains scores, I can say that I did all 4 of the above because... they made sense.
measure numbers, same system and same measure number, large and clear (all the score anyhow) and ample marginal space.
I should also note that, while for printing it IS inconvenient and costs more, I went for landscape format instead of portrait: It's much better in the eye of the duet, sitting on the same bench, etc. Tried the portrait version, and couldn't work with it as a pianist!
Ahhh... why not?
Here's a couple of pages from the first work in my work. Keep in mind that page 2 (the first link) doesn't have a page number, exactly because it's the very fist page of a 40 page score!)http://www.musica-ferrum.com/images/ps2.jpg