Very often it is convenient for the organists to divide their music so that the left page is played by the organist performing the 2nd part and the right page - for the 1st part.
Personally for me, such division of the music makes it really difficult to follow the music of the partner. There are more chances that one organist would lose the beat and consequently the duet will fall apart. For this very reason it is quite necessary sometimes to look at another organist's page and try to find the right place in the music.
Because of this simple reason I find that the easiest division of the music on the page is when both parts are visible for both performers and placed one under another. This way the entire musical view can be seen.
To make music together for 4 hands (and feet) is very fun thing to do. It is possible to play not only the original compositions for organ duet, but also arrange choral or orchestral music because when you have 2 performers, technically there are twice as many possibilities. It is also possible to improvise.
Because 2 performers sit on the same organ bench, both can very often turn pages and change organ stops on each other's side of the instrument. Of course, you need to feel your partner well because otherwise both people could try to turn the pages or pull the same stops at the same time which would make a lot of chaos for the entire performance.
Listnen now to the Sonata No. 1 in C major (published in 1781) for a duet by the classical style German composer Franz Seydelmann (1748-1806). In these videos, you can see the four hands very well. I performed this piece with my wife Ausra as a part of the organ demonstration of Pinkevicius Organ Duo on the famous organ by Adam Gottlob Casparini (1776) which stands at the Holy Ghost (Dominican) church in Vilnius, Lithuania.
Part I - Allegro
Part II - Andantino
Part III - Allegro di molto
By the way, do you want to learn my special powerful techniques which help me to master any piece of organ music up to 10 times faster? If so, download my video Organ Practice Guide.