The install docs say a free breaker is preferred of available, and many breakers are not rated for multiple conductors per terminal.
I brought this up when I installed my Sense and support mostly wrote me off as a loony who must not know what I’m talking about. Even the breakers in the Sense ads and physical displays are Square D Homeline breakers which require 14 GA or larger, and are the same model in my own home.
Sure you can jam them into an occupied breaker and they feel mostly snug on a breaker of there own, but when it comes to electricity I’d rather not risk a conductor coming loose and causing an arc. You can pigtail them to some 14GA and then into an unoccupied breaker or into one rated for two conductors per terminal.