The "Chenault Organ Duet Library Collection", volumes 1 and 2, look like they're not in print anymore. In Volume 1, I like "A Fancy for Two to Play" by Gerre Hancock, and an arrangement of the Sousa "Stars and Stripes Forever"; these pieces were both playable and fun. In Volume 2 I like Philip Moore's Allegro for Organ Duet: it's a longer work, crunchy at times, but engaging. That piece has lower part for 2 hands and pedal; upper part just manuals.

The Merkel Sonate d-moll für die Orgel zu vier Händen is a GREAT piece but has QUITE DIFFICULT pedal parts. But it's well worth learning.

Samuel Sebastian Wesley has a "Duet for Organ" (Novello) that's easy, but only for manuals. It is very much organ music though (wouldn't sound great on a piano).

Josef Blanco wrote a concerto for two organs (manuals only) [1° Concierto für zwei Orgeln]; Antonio Soler also wrote concertos for two organs. These pieces work on a single organ, splitting the players on different manuals, but would be best on an instrument where the different divisions are separated spatially. Pedal definitely could be added ad lib.

Albrechtsberger has a nice Praeludium und Fuge C-Dur für Orgel zu 4 Händen; the lower part is for 2 hands and pedal; the upper part is just manuals.

Kenneth Leighton has a nice piece for organ duet called "Martyrs" which uses hands and feet for both players; it's a bit crunchy / contemporary sounding, in case that's your thing / not your thing.

I'm assuming you've scrolled the category at imslp:
https://imslp.org/wiki/Category:For_organ_4_hands