Hi there – while you can’t add HTML or CSS directly to blog post titles – i.e. to add a line break and formatting – what I’d suggest instead is to add the post subtitle in the body of your blog post, with a heading formatting tag like h3 (Heading 3) or h4.
Here’s a guide to adding heading formatting in the Visual editor:
https://en.support.wordpress.com/visual-editor/#styles
Let us know if you need further help.