"now", the doctor says. "I get to choose which one of you to change."
"Just one of us?" Said Susan a bit surprised.
"Yeah", Bill replied. "me and my girl stick together."
"You see", The doctor said. "Some experiments are best if you observe a couple where only one person is changed. To see how a normal adapts to a changed person. Besides way too many of my colleagues due mutualistic changes of couples. Only so many times you can swap parts on a couple for example, and have a mutually heterosexual though scrambled result before it gets old. If I change only one of you it'd be far more fascinating and I'd be exploring something new and relatively untouched. Don't worry you'd both be paid, by interacting with the changed person you are part of the experiment. And I know just who I will change."