Play the polyrhythms divided by both hands to hear what is going on.
In the case of the Faure Nocturne where the middle voice is syncopated quarters, practice by pulsing in eighth notes (play two eighths when you have a quarter, similarly replace longer values with eighths, and play tied notes), also play a note where you have rests. You do this in order to hear the underlying rhythmic grid. Gradually restore rhythmic values to those as written.
Also since the Nocturne contains polyrhythm passages in 3 voices, practice all permutations of 2 voice combinations. That is: 1+2, 2+3, and 1+3. Do this before you combine all the voices. Apply this procedure to the rhythmic grid exercise above.
Another thing to practice is the rhythm as written, removing all pitch values. You can tap, clap, stomp, beat drums, or use fingers on a closed fallboard or desk surface.