"Because it puts two identical personalities on one plane." The man
with Horning's face dropped into a chair and hunched forward. "Take our
own situation as an example. You're married to a shrew, a termagant.
You want to leave her."
"Yes."
"I, on the other hand, have a young and charming wife who holds a
considerable fortune in her own right. Consequently, it would be
ever so much to your advantage to switch places with me." Horning's
counterpart brought up one square-knit hand in an expressive gesture.
"What's to prevent your murdering me and moving in?"