Just mentioning to cover all the bases: you’ll want to make sure you’re using the correct multi-value YAML syntax in your front-matter when you include multiple authors (eg. single line or multi line array). I like using the Linter plugin to help me out with this.
<pre style="background-color:#ffffff;">
<span style="color:#323232;"># single-line array
</span><span style="color:#323232;">author: [Sam Jones, Mary Apple]
</span><span style="color:#323232;">
</span><span style="color:#323232;"># multi-line array
</span><span style="color:#323232;">author:
</span><span style="color:#323232;">- Sam Jones
</span><span style="color:#323232;">- Mary Apple
</span>
Once you have something like the above, using flatten should enable returning results for either author; with one test file in my vault, this worked for me: