How to add a second featured image

Hello! Thanks again for your help creating the child theme! I have a new questions ... I would like the capability to add a second featured image. For a "POST" I would like the current featured image to remain as a thumbnail. When a visitor clicks to view the post, I would like the header to be the second featured image. Where would I go to make those changes? functions.php? style.css? I'm just starting to learn this whole .php stuff ... so ... initially if you could just point me in the right direction, I sure would appreciate it. But don't be surprised if I ask for more help ... lol. Thank you. Chris