Schubert is a central character in the piano four hands repertoire. Yet he is not the first. Out of all the major composers, that honor befalls Mozart, when he wrote his Sonata for two pianos in D major, K448. His interest in the genre (if you can call it that at the time) might have come from his frequent playing together with his sister Nannerl.
Influential composers like Haydn and Beethoven didn’t write anything for piano four hands. It was indeed Schubert who did the most. He wrote more than thirty works, spread out over his life. Then there was Ravel’s Ma mère l’oye and Rachmaninov’s Six Morceaux.
After the horrors of the Frist World War it seems the frivolity no longer had a place. For the next decades music needed to have a more serious tone. There were some examples. I’m currently listening to Visions de l’Amen written by Olivier Messiaen in 1943. Even in 2020 Argentinean composer Juan María Solare created a tango like work for the genre.