Can't update the second front end
Drush uses the database to determine what versions of modules are installed; therefore, Drush believes the second frontend to be up-to-date, even though the code is still at an older version.

To update your site:


Do not update live. Make a local dev environment by copying one of the frontends and the database.  Update the local dev environment
After updating and testing the local system, push the new code to the live systems.
Run drush updatedb on both frontends