Both of my males (Bogey and Doodlebug) are also neutered and will try to hump each other. Doodlebug will also try to hump Pebbles who wants nothing to do with that nonsense.
Bogey even humps the air when the play gets too rough. There are some days I was want to
But it always happens during play and I try to just distract and redirect them with either a toy or treat. If your boys are not both neutered my suggestion is that you do this since it will lead to marking inside you home.