This tutorial provides the code which enables you to add a 2nd featured image to any post. You can then display the image side by side, inline with your default featured image like you see in the following screenshot of the Brunch Pro themes archive page:
You can also use this solution to display more than 2 featured images on any blog listings page, including archive, author, blog, category, search, and tag pages as well as single posts and pages like the following example:
Shows the addition of a 2nd meta box for uploading another featured image as well as how the images display on different screen sizes.
The solution has been tested on the Brunch Pro child theme by Shay Bocks which is included in the StudioPress Pro Plus themes package and includes CSS for Media Queries which work in this theme. The code may need tweaking for use in other themes.
Here’s the code for logged in members: