Then the kohen shall order, and the person to be cleansed shall take two live, clean birds, a cedar stick, a strip of crimson [wool], and hyssop. [Metzora 14:4]
Rashi explains that since tzara'as comes about because of lashon hara, the person being purified must bring two birds, for birds ''constantly twitter with chirping sounds''.
The Talmud Yerushalmi [Berachos 1:2] cites the words of R' Shimon bar Yochai: ''If I would have been standing on Har Sinai at the time the Torah was given to the Jewish people, I would have requested before Hashem that He create two mouths for man. One mouth would be for the purpose of toiling in Torah study, and the second would be for the purpose of allowing him to speak about his ordinary needs.''
Later, R' Shimon bar Yochai changed his mind, and he said: ''If the world cannot withstand man's slander when he has only one mouth, how much more so would this be the case if he had two mouths.''
Source: Rabbi Yisrael Bronstein