Season 4, Episode 16 - The Cohabitation Formulation
Aired February 17, 2011
Raj's sister, Priya, is in town again and rekindles her relationship with Leonard, prompting Penny to reconsider her feelings for him. When Bernadette asks Howard to move in with her, he is forced to choose between his girlfriend and his mother.
Season 10, Episode 4 - The Cohabitation Experimentation
Aired October 10, 2016
After Amy's apartment is flooded, Penny suggests that Amy move into Apartment 4A with Sheldon. Unsure about finally cohabiting with his girlfriend, Sheldon comes around to the idea when he looks at it as a five week experiment in living together. Sheldon suggests they move into Penny's old place, Apartment 4B, so they would know if he's comfortable because he's living with Amy and not just comfortable in his own apartment.
Meanwhile, Howard and Bernadette decide not to learn the gender of their baby, but they question their decision when Raj reveals that he knows the sex of the baby.