Twinkie isn't a cake, it's another term for "someone you have in common with." Because the Twinkie cakes come in packages of two, well, two people come in packages of two, as well. So if you and someone has a lot in common, that person becomes your twinkie!
Twinkies one: I like to swim.
Twinkies two: omg, me to!
Twinkies one: We are so totally twinkies!
Twinkies two: Omg!