filter_pre_get_posts help

confused.php

function filter_pre_get_posts(&$query) {
  if ( $query->is_home AND $query->is_main_query() ) {

    $query->set('meta_key', 'my_key');
    $query->set('orderby', 'meta_value_num date'); 

    // should sort by meta-value as number, then by date 
    // actually sorts by date and ignores anything with my_key set




    $query->set('orderby', 'meta_value_num'); //works as expected (sorts by my_key)
    $query->set('orderby', 'date'); //works as expected (sorts by date)
  }
}