Tim is a typical two-headed worm creature with his right head dominating his left head.
In the episode "Better Off Undead", when the right head threatens the left in Mark Lilly's class, Mark makes the decision of separating the two through surgery with the left getting the bigger part of the body together with the junk. Left Tim, with the bigger body part, faces no real adverse effects from the separation whilst Right Tim is left weak and slowly dying on life support.
When Randall Skeffington has left to join a zombie cult, the left side of Tim moves in with Mark and becomes too attached to him. After left Tim gives birth to a baby head and asks Mark never to leave him, Mark puts the two parts of Tim back together again to get rid of the left one.