Hello people!
Please, who has idea how to play this in D-major and C-major at the smae time? Should I transpose one of the music lines? Thanks!

file:///C:/Users/Loby%20bar/Downloads/[Free-scores.com]_traditional-gloria-in-excelsis-deo-trad-french-christmas-song-27100.pdf

As the melody is written out for clarinet you will have to transpose it down into C to match the piano part; or move the piano part up into D. With such a common carol I'd have though it should be possible to find a version for piano or a choral version.