query_posts() with multiple meta data comparisons

First, do not query_posts; use WP Query instead. That being said, I assume that pce_monday_open is lower than pce_monday_close and that you want posts between pce_monday_open and pce_monday_close, so I think your comparison logic is wrong. Also, you may need to set the relationship between the two meta query arrays:

$pce_arg = array(
    'cat' => 6,
    'meta_query' => array(
        'relation' => 'AND',
        array(
            'key'     => 'pce_monday_open',
            'value'   => $pce_time,
            'compare' => '<=' ),
        array('
            key'      => 'pce_monday_close',
            'value'   => $pce_time,
            'compare' => '>='
        )
    )
);

$query = new WP_Query( $pce_arg );

if ( $query->have_posts() ) {
    while ( $query->have_posts() ) {
        $query->the_post();
        //Loop on each post
    }
} else {
    // no posts found
}

wp_reset_postdata();