How could you come into contact with them again if you were in different universes? If your universe constantly communicates with theirs in order to know when to combine (when you begin to occupy adjacent corresponding space), in what sense do you occupy 'different universes'? How do you know there isn't just a wall that appears whenever you can't measure it - and how do you know it's there if you can't by definition measure it? (Note: only ask him these questions if you want to get back at him for confusing you.)

Also The Simple Truth, that one's good too.

Edit: I'm pretty sure the relevant bit is

Inspector Darwin looks at the two arguers, both apparently unwilling to give up their positions. “Listen,” Darwin says, more kindly now, “I have a simple notion for resolving your dispute. You say,” says Darwin, pointing to Mark, “that people’s beliefs alter their personal realities. And you fervently believe,” his finger swivels to point at Autrey, “that Mark’s beliefs can’t alter reality. So let Mark believe really hard that he can fly, and then step off a cliff. Mark shall see himself fly away like a bird, and Autrey shall see him plummet down and go splat, and you shall both be happy.”

We all pause, considering this.

“It sounds reasonable…” Mark says finally.

“There’s a cliff right there,” observes Inspector Darwin.

Autrey is wearing a look of intense concentration. Finally he shouts: “Wait! If that were true, we would all have long since departed into our own private universes, in which case the other people here are only figments of your imagination – there’s no point in trying to prove anything to us -”

A long dwindling scream comes from the nearby cliff, followed by a dull and lonely splat. Inspector Darwin flips his clipboard to the page that shows the current gene pool and pencils in a slightly lower frequency for Mark’s alleles.

(Emphasis mine.)