This exhaustion would force Bill out of Dipper's body and into one of Mabel's puppets,

Thankfully, because Dipper's a spirit, he's able to possess one of Mabel's actual puppets to convince her that Dipper she'd been talking to before was Bill all along.

That's why the show takes place during the last summer of Dipper and Mabel's childhood,

Dipper and Mabel Pines, spending the summer with their grunkle stan in the sleepy town of Gravity