What s a good title for a story about twins where one walks down the path of evil and the other joins the forces of good to stop them?

Don't. Just Don't.

Cheesy twin trope story #124321

Vision

Since the actual story would be relevant in what the title should be, it would be difficult to name your story for you. You might write the story and after you have completed part of it, the title may just "come to you".

Get the story underway, then consider titles.

Paper Dolls.

Have them conjoined twins separated at birth.

Divarication