Merge 2 args in one WP_Query and order it by date

A better approach could be using three queries. First two query retrieve the post ids, and third one query post by ids.

// first query
$first_ids = get_posts( array(
    'fields'         => 'ids',
    'posts_per_page' => '10',
    'post_status'    => 'publish',
    'post_type'      => array('news','partners'),
    'orderby'        => 'date',
    'order'          => 'DESC'
));

// second query
$second_ids = get_posts( array(
    'fields'         => 'ids',
    'posts_per_page' => '10',
    'post_status'    => 'publish',
    'post_type'      => array('post'),
    'orderby'        => 'date',
    'order'          => 'DESC',
    'tax_query'      => array(array(
        'taxonomy'       => 'tax',
        'field'          => 'term_id',
        'terms'          => array(5)
    ))
));

// merging ids
$post_ids = array_merge( $first_ids, $second_ids);

// the main query
$query = new WP_Query(array(
    'post_type' => 'any',
    'post__in'  => $post_ids, 
    'orderby'   => 'date', 
    'order'     => 'DESC'
));

if( $query->have_posts() ):
    // here you go
endif;