WP Sort by multiple meta's

custom-loop.php

<?php
$WP_categorias_filtro = array(
    'post_type'       => array('proposta', 'problema'),
    'posts_per_page'  => 6,
    'meta_query'      => array(
        'relation'    => 'OR',
        'contagem'    => array(
            'key'     => 'contagem_views',
            'type'    => 'NUMERIC',
            'compare' => 'EXISTS'
        ),
        'data'        => array(
            'key'     => 'contagem_views',
            'type'    => 'NUMERIC',
            'compare' => 'NOT EXISTS'
        ),
    ),
    'orderby'         => array(
        'contagem'    => 'DESC',
        'post_date'   => 'DESC'
    ),
    'paged'           => $paged
);