The literary device being used is a proverb and when the Nurse quotes it, "Is your man secret? Did you ne'er hear say, Two may keep counsel, putting one away?", she is asking Romeo if his manservant can be trusted. When Romeo aasures her that his man is as true as steel, she tells him the proverb above that says two can keep a secret, but not three.This tells me that the nurse felt the secret should stay between the two of them and no one else.
Reference: The Language and Literature Book by McDougal Littell
It sounds to me like a proverb - the Nurse is asking if Romeo's man can keep a secret, reminding him that two people can keep a secret as long as one of them is "put away," or got rid of somehow. In other words, two people can't really keep a secret.
Here is the definition of "proverb" given at eNotes:
"Proverb - a short saying, usually of unknown or ancient origin, that expresses some useful thought, commonplace truth, or moral lesson and is most often expressed in simple, homely language"
You can check the link below for more information about this literary device. Good luck!