Drush within Multisites
you can use:

drush dl modulename --cache


From drush help dl:

Download Drupal core or projects from drupal.org (Drupal core, modules, themes or profiles) and other sources. It will automatically figure out which project version you want based on its recommended release,
or you may specify a particular version.

...

Examples:

 drush dl webform --cache                  Download webform. Fetch and populate the download cache as needed.

Options:
 --cache                                   Cache release XML and tarballs or git clones. Git clones use git's --reference option.                                                                         
   --cache-duration-releasexml             Expire duration (in seconds) for release XML. Defaults to 86400 (24 hours).
...