Headroom guidelines are generally the same for 2.40 movies. Rule of Thirds also can be applied. It's just that being such a wide aspect ratio, it lends itself naturally to more modernist compositions that use large areas of negative space. But one can also compose 2.40 very classically, balanced.
For example, the wide shot here in "Superman" balances the two figures on the left with the farm house on the right:
But the singles are pushed farther to the edges than the Rule of Thirds, and doesn't use over-the-shoulders, as a way of creating visual isolation that foreshadows the actual separation of the two characters, enhancing a feeling of loneliness: