The following code for a landing page — without header, original sidebars and footer and with new sidebars added — goes in custom_functions.php. This code does the following to pages with a custom field named “landing-page”.

adds a body class of “landing-page”
removes the header, sidebars, and footer
registers 2 new sidebars called Landing Page 1 and Lading Page 2
builds the code for the new sidebars (note that an extra and are added because there’s no hook in between the end of #content and the end of #content_box)
then implements all of the above before Thesis loads. If you want a sidebar | content | sidebar arrangement, you’ll need different markup.