As a former graphic designer (of 25+) years, I know the importance of short, pithy titles. You should definitely try to keep titles to one line. A full sentence isn't necessary, and 2 lines will lose a reader. On the Internet, remember that your title will be shortened to the search engine's maxium length, which means your subtitle may not display at all. Also, you want your lead (the opening lines of your article) to display enough to hook the reader. Again, Internet search engines impose a maximum number of characters which will display from your lead. So, your lead needs to grab the reader's attention in the first few words.