Show featured image if no secondary feature image

Love the plugin. In my theme the featured image of a post is used in my slider and then also displayed in the post. I want to be able to display a different image in my post and I’m accomplishing this using your plugin. I also need to write a conditional statement that says if there is no secondary featured image, then use the post thumbnail. Can anyone assist me with that little block of code?

Thanks
Keith

https://www.ads-software.com/extend/plugins/multiple-post-thumbnails/