The full entry needs to be in one paragraph to show up as one entry in the Table of Contents. The issue then becomes how to break it onto two lines in the book itself.
Personally I just usually use forced line return in the book and then fix the Table of Contents line breaks after generating it with a search and replace to take out forced line returns.
Other options exist, such as changing the paragraph settings to force the text onto a second line automatically (with right indent or some such thing). If your chapter titles are in a text box of their own, adjusting text box width manually to force the title onto two lines is another possibility.