Query Posts | Combining multiple form inputs into query arguments to generate one filter result

I needed to make the array of categories separate to each other, as this is where the ‘AND’ is being applied:

/* Filter Portfolio Post Results */
function portfoliofilter_filter_function(){
    $args = array(
        'orderby' => 'publish_date', // we will sort posts by date
        'order' => 'DESC' // ASC or DESC
    );

    // for taxonomies / categories
    if( ( isset( $_POST['sectorfilter'] ) ) || ( isset( $_POST['categoryfilter'] ) ) )
        $args['tax_query'] = array(
            'relation' => 'AND',
            array(
                'taxonomy' => 'category',
                'field' => 'id',
                'terms' => $_POST['sectorfilter']
            ),
            array(
                'taxonomy' => 'category',
                'field' => 'id',
                'terms' => $_POST['categoryfilter']
            )
        );

    $query = new WP_Query( $args );

    if( $query->have_posts() ) :
        while( $query->have_posts() ): $query->the_post();