Unfortunately only one author can be assigned for each post, this is the author that will be displayed on modules and in the post page.
There could be plugins for this case, but that will require modifications in the them files most likely.
Yes, I found a plugin and was able to modify the template a bit. The only issue is that while I got it working under the title, for some reason I can’t get it to work with the author box. I can get multiples of the original author, but can’t figure out how to add in a second.
The author box both on the post page is meant to display only one author, It would be difficult to modify it to display multiple authors. This is where it is defined in the theme, but it will most likely require lots of modifications to behave as you like
Maybe you can disable it and use a plugin to display a more advanced author box which has the options you need, if there are such plugins.