YA twins who share the same body, and transform from girl to boy once a month. Cover: half blue figure, half pink figure. Read around mid-2000s.

[Becca]

Cycler by Lauren McLaughlin?

[Andy]

Just what I was thinking.

[Kris]

Tanarah, Cycler by Lauren McLaughlin - Becca and Andy's suggestion - sounds like your book.

"...the four days Jill is out of school each month, she is not Jill at all. She is Jack, a genuine boy - complete with all the parts. Jack lives his four days per month in the solitude of Jill's room."

Here's the short Amazon "Look Inside" preview - https://www.amazon.com/Cycler-Lauren-...

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