Here are the 11 movies that Walter Matthau and Jack Lemmon appeared in together. Please note that "Kotch" was directed by Lemmon and only had a cameo. Also they never actually appeared on screen together in "JFK".
Obvious ones left out.
Film
warner movies
Movies which have entries listed on https://scarytaskbar.com/movies/
Lemmon & Matthau
by Wambie