Shakespeare Programming Language, 204204 156 bytes
-48 bytes thanks to Jo King (mostly by changing the output method)
A.Ajax,.Puck,.Act I:.Scene I:.[Exeunt][Enter Ajax and Puck]Ajax:Open mind.Puck:Open
mind.Is I nicer thanworse zero?If not,Let usScenesoSpeak Vthy.Is you as big as I?If so,LetsoLet usAct I.You
is zero.Scene V:.Puck:Speak thy.
Outputs the null characterExits with error if the input is not double speak, and outputs nothing (exits with error)warning if it is not double speak (which is allowed by default).