Stand in

"My name is Cathy!" 
Lyle bursted out in a laugh. It sounded so ridicolous hearing himself saying that from a copy of her body.

"Perfect! You nailed it! You sound exactly like me!", the real Cathy encouraged.
"Don´t laugh at it though. You are doing fine. But your body language is atrocious. We will need to start practicing my movements right away so you can pass as me at work when I go on vacation".

Lyle nodded. He was out of work at the moment when Cathy, who had majored in the same field of work as him,  had asked him to be hers stand-in. Her job was burning her out and she sorely needed some time off, but her boss had threatened to cut her loose from her high-paying job if she did.
"Why can´t you use the spell book to make me more like you?", Lyle wondered.
Cathy sighed. "It is kinda all or nothing. If I would cast a spell on you so you would be like me I would effectively create my own clone. I like you too much to do that, it would like be killing you".
"Oh".

Cathy went on vacation and came home a month later. Lyle had grown accustomed to Cathy´s life back home and loved every minute of it. So Lyle used the spell book to transform Cathy into his old body and cast a second spell to make Cathy into Lyle both in body and mind.
The new Cathy took a liking into the new Lyle and now they are expecting their first child within three months.