I'm working on a site with multiple authors, and am trying to set up Google authorship via rel tags. Since it's a multi-author site, the links to the authors Google+ profiles will be on their profile page. However, the profile page allows them to have links to Facebook, Twitter and LinkedIn profiles as well. My question is, if I add rel="author" attributes to all these links, will Google still parse the authorship information? Or do I need to set it up to only have the rel="author" attribute on the Google+ link? Thanks for your help.
This has already been answered on this site: How to implement rel=author on a page with multiple authors?
It would appear that you cannot have multiple authors, as Google won't accept it for that article, and then Google will view your entire site as not implementing the author markup correctly.
The moral of the story is to be very, very careful when implementing rel=”author”. Don’t [anger] the Google Gods by having more than one author attributed to a page (like I did), as it may negatively impact the authorship of your whole site, not just the relevant/affected pages.
...I don't know that Google has developed the authorship markup system to account for that situation. When I searched for references to that scenario, the primary Google docs simply don't mention multiple authors for one piece of content. Furthermore, people who have tested this scenario on their own note that the first rel=author identified (out of many) is the one the Rich Snippets Testing Tool notes as verified.