Co-Authors Plus is a neat plugin that makes it possible to have multiple authors for one post in WordPress. This is a useful feature WordPress doesn’t yet support.
Relevanssi only picks up the first author from the post, but making Relevanssi find the other authors as well is very simple, thanks to good design choices made by the Co-authors Plus developers. The plugin stores the extra authors in a custom taxonomy. Indexing custom taxonomies is elementary for Relevanssi.
In order to make Relevanssi index all authors for posts, simply add “author” to the “Choose taxonomies to index” option and re-index.