I contacted HubSpot support regarding this exact idea. The following was their suggested solution.
“At the moment it’s not possible to have 2 blog authors to one post. As a workaround, you could add additional text to the post to state the additional contributors and hyperlink their names. This will make it visually similar to the name of the blog author.”
I work for a CPA firm and am in charge of implementing a new firm-wide team marketing effort where we’re teaching our staff to develop themselves as thought leaders. It’s therefore important to us that all our contributing staff can be listed as authors on the posts the co-author. I’m hoping this becomes an option in the near future.