What is 2's?
Sharing a ciggerette between two people.
Smoking only half a ciggerette so there is enough for another person to smoke, or waiting for someone to smoke half a ciggeret then finishing it off.
Person one: "Can I have a Ciggerette?"
Person two: "No, I only have one ciggerette left. But I can save you 2's on it."
Person one: "Thanks mate"