Below are the steps for sharing the files directory between different instances:
Create Target Directory:
sudo mkdir /home/drupal/files
Go to default Drupal sites/default directory or specific site directory path in multisite
Create symbolic link:
ln -s <target-link> <sourch-link> ln -s /home/drupal/files files
Now you can access all the files uploaded via Drupal admin interface can be viewed in /home/drupal/files.