She often has to listen to one venting about the other when the other isn't listening. Fluttershy never liked being their therapist and often resorted to shoving the two of them in a closet to make up for whatever the other did ("You ate my last bagel, it even had my name on it!" "So what, don't you always buy more?" "BOTH OF YOU SHUT UP JUST APOLOGIZE NOW")
Many people see Fluttershy as a kind and sweet angel but when it comes to Zephyr and Lightning…they would rather not get in her way. Also, don't you dare mention Rainbow Dash around the two of them. Zephyr's swooning and Lightning's hatred was never a tolerable mix…