At this time, Atticus doesn't support manually creating multiple lines in a title or subtitle or adding formatting to a portion of these elements.. However, you can customize a title page to fit your brand and style.
If you don’t like the look of the auto generated title page you can delete it (click the little x beside the title when you hover over it) and replace it with a new page that matches the theme you’ve chosen for your book.
You can either simply by Add a New Chapter with your details and drag it to the Front Matter section of your book, or you can even insert a full page image and insert a designed image of your book cover, if that works better for you.
You can learn more about these techniques in this tutorial: https://www.youtube.com/watch?v=FMu4-jkIoRY