Primo and Secondo are on separate pages or together? Do you start them one way or the other or does it matter little? Which do students find less confusing?

I think that duets (on the same piano) are always on separate pages and piano duets (on two pianos) are on the same page/system.

If anything I would assume that it would be easier for a beginner to see just his/her two staves and play as they always play, but listen something additional on the back, rather than see two extra staves bellow his/her and having to decide what to play, how to 'gap' these two and move on to the next two and so on...

This is my understanding as well as far as how music is generally notated. If you have music that is not on separate pages for Primo and Secondo, then I'd photocopy and cut and paste them so that they were that way. I think it's very important since kids are not used to reading on a multi-staff system.

As a student with about 3 yrs experience I can tell you that seperate pages is MUCH less stressful! No doubt about it.

As a student, I'm ok either way. Same page makes it easier to see what is going on for the other player--"Oh! He has a rest here--I need to keep playing!"

I think as a teacher, I like to see two scores together in one page so that I know what is going on for both players in my studio. However, I think my students response better when they are in separate page because it is less confusing.
Conclusion, I just hope that all the publishers will publish the duet in two version:
1. Secondo and Primo in separate page for easy use for students, and
2. Secondo and Primo in same page for easy use for teachers.

Just like the orchestra score, it is easier for the conductor to read the conductor score with all the instruments, but easier for orchestra player to see only his or her part on the page.

That is my opinions.

I cannot STAND it when duet parts are written on the same page. It seems unnecessarily confusing and even I (the teacher!) find it difficult to read sometimes. Unfortunately a lot of beginner/elementary duet books seem to be laid out this way. I have numerous duet books that I just haven't used yet as it would definitely require some photocopying, cutting and pasting before I'd be happy to present it to a student.

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.

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.

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)

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
and
http://www.musica-ferrum.com/images/ps3.jpg

Measure numbers are essential in any case

I guess, players who could get through the music perfectly without rehearsal might not need them, but in my house, we need to rehearse, and so we need measure numbers.