Including template footer.tpl.php within another template page.tpl.php
The way to include another .tpl.php file is the include function.

Another idea is to add your footer as a region in the page.tpl.php and assign a region wise theme as defined in Drupal 7 Template (Theme Hook) Suggestions


  region--[region].tpl.php 
  The region template is used when a page region has content, either from the Block
  system or a function like hook_page_build(). Possible region names are determined by the  > theme's .info file.