How to share same post to multiple site in wordpress?

You can use the function switch_to_blog() for this

$other_id = 1234 // the id of the other blog to save the post to
switch_to_blog($other_id);
$my_post = array(
  'post_title'    => $post_title,
  'post_content'  => $post_content,
  'post_status'   => 'publish',
  'post_author'   => $post_author,
  );

// Insert the post into the database
wp_insert_post( $my_post );

restore_current_blog();

There is a pitfall if you execute this code on the save_post hook, because wp_insert_post also calls save_post and you end up in an infinite loop. This post on Stack Overflow gives a solution for that.