only one author showing up w/ twenty eleven theme

This seems to be a problem that happened when I upgraded my version of WP, but now only one author is showing up when I’m using the twenty eleven theme (it did before the upgrade). When I go to the author pages the posts show up for all authors, even if their name isn’t visible, so must just be a display problem. I’ve seen some posts on the forum about displaying multiple author boxes but I need a fix to get the two authors to show up at the top, e.g. “written by X and Y.” Thanks

http://wordpress.org/extend/plugins/co-authors-plus/