Since our August sprint release, we can now treat second (or third, or fourth) authors in the same way as the primary author. This means that by adding a second author to your post this content will now appear on their profile. These posts can also be pulled to other locations by the JQuery plugins using their userId. They will also receive the same uplift to their Expert Influence as the primary author.
Co-authors will be listed as before (see the top left of this post) but the content will not be classed as 'their post' and so it will not be listed on their profile page (nor appear in their statistics).