I was just wondering the same thing. Apparently not by any settings. But you can change the plugin css to do that. Then it will be always centered. In my opinion it looks better cetntered anyway. Just edit
twentytwenty/public/assets/css/twentytwenty.css
and add
margin-left: auto;
margin-right: auto;
into the .twentytwenty-container {} part. Hope this helps.
