When apache has several Require lines in a section it's enough that one of these lines match for the user to be authorized.
The following will authorize the user SumeUser or any user in group 5 or 6. However, when editing the directive from webmin the first Require line will be removed.
<Location /stats>
Require user SomeUser
Require Group 5 6
</Location>
Similarly it's possible to have several users on multiple lines in the configuration, but Webmin will only use the last line:
<Location /stats>
Require user User1 User2
Require user User3 User4
Require user User5 User6
</Location>
will be turned into
<Location /stats>
Require user User5 User6
</Location>
by Webmin.