I know I can add users and I already have the Co-Authors plug-in and it's letting met add two authors but only one is displayed on the page.
for reference, this is what displays at the top of my post, but I need another author to show up as well
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I am unfamiliar with the original. They may also have something similar to the new "Co-Authors Plus". You would need to review that code for "apply_filters", "add_actions" or just global functions like those in "Co-Authors Plus".
Meanwhile, you might investigate migrating to the new version of "Co-Authors Plus". They offer a template tag, and pretty detailed instructions for how to use it.