The Twins Thing is Phil and Lil's favorite stuffed animal who acts like them, follows them everywhere (Phil and Lil sometimes carries them), and is built-proof (water, mud, etc).
Getting ready for Family Reunion (hosted by Phil and Lil and some of their ansentors) party, a stuffed animal who looks like a smaller verison of Phil and Lil was in Kimi's bag, and was thrown out onto a tree by a lake and hanged onto a broken branch, holding on. Tommy and his friends saved him. During the party, Betty DeVille introduced the Twins Thing to himself and Phil and Lil and Twins Thing says "pee pee" which is what Phil and Lil's other favorite activity is. The three of them hugged, leaving Betty decided to have Phil and Lil have them.
The Twins Thing is a smaller version of Phil and Lil as plush dolls.