how to Assign Multiple Roles for a user in wordpress?

Yes, WordPress allows this. You can do it in your code:

    $someone = new WP_User( $user_id );
    $someone->add_role( 'role-1' );
    $someone->add_role( 'role-2' );

You can also use a plugin such as Multiple Roles. The plugin will transform the select dropdown into checkboxes.

WordPress — Role selection checkboxes

For completeness, this is possible!

 $theUser = new WP_User($userId);
 $theUser->add_role( $role );

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