Get post by two meta key but order by one of them

use

'orderby' => 'meta_value_num',
'meta_key' => 'post_views',
'order' => 'DESC',

instead of

'orderby' => array( 
        'post_views' => 'DESC',
        ),

your way is okay for string, but when you use number it is better to use meta_value_num

meta_value will sort number weird 1, 3, 34, 4, 56, 6, etc, rather than 1, 3, 4, 6, 34, 56 as you might naturally expect as codex explain