E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Like the two kings of Brentford smelling at one nosegay. Said of persons who were once rivals, but have become reconciled. The allusion is to an old farce called The Rehearsal, by the Duke of Buckingham. The two kings of Brentford enter hand in hand, and the actors, to heighten the absurdity, used to make them enter smelling at one nosegay (act ii. s. 2).