Show posts from two specific category in WP_Query

So you want to show only posts that have both employee and full-time categories.

If that is the case than you can do the following.

Because you haven’t posted the full query args I will only show the tax_query part

'tax_query'         =>  array(
    'relation'      =>  'AND',
    array(
        'taxonomy' => 'category',
        'field'    => 'slug',
        'terms'    => 'employee',
    ),
    array(
        'taxonomy' => 'category',
        'field'    => 'slug',
        'terms'    => 'full-time',
    )
)

This should do the trick