Get entries relatedTo multiple categories
The relatedTo parameter's targetElement property has to be an ElementCriteriaModel or a single category model:

{% set featured = craft.categories.group('myCatGroup').slug('featured').first() %}
{% set caseStudies = craft.categories.group('myCatGroup').slug('case-studies') %}

{% set entries = craft.users({
    group: 'resource',
    relatedTo: [
        'and',
        { targetElement: featured },
        { targetElement: caseStudies }
    ]
}) %}