Sorry about the multi posting, but I figured I would also do a post outlining some information that may be helpful in solving this:

This is a completely new install. I installed Zencart in a folder called NewSecure

I added 2 templates. One called fatvanish and another called musclenow. I added the template_info to each of those folders and in the fatvanish the $template_name was fatvanish and in the musclenow the $template_name was musclenow

I added 2 files to includes/config_sites/. One was called newsecure.fatvanish.com_config.php and one called newsecure.musclenow.com_config.php. I added newsecure.fatvanish.com and newsecure.musclenow.com as subdomains pointing to the NewSecure folder where I installed ZenCart.

Below is the contents of newsecure.fatvanish.com_config.php

<?php
define('DIR_WS_CATALOG', '/');
define('DIR_WS_HTTPS_CATALOG', '/');
$template_dir = "fatvanish";
define('SITE_NAME','fatvanish');
?>

Below is the contents of newsecure.musclenow.com_config.php

<?php
define('DIR_WS_CATALOG', '/');
define('DIR_WS_HTTPS_CATALOG', '/');
$template_dir = "musclenow";
define('SITE_NAME','musclenow');
?>

In the config.php files I am using http://newsecure.musclenow.com as the http_server and https://celia.webserversystems.com/~musclewi/NewSecure as the HTTPS_SERVER (this is a shared server).

As stated earlier I'm still not 100% sure if

include_once('includes/config_sites/sites_switch.php');

issupposed to go before or after

<?php

in includes/config.php

I'm currently getting either a blank page if going to http://newsecure.musclenow.com or http://newsecure.fatvanish.com despite the fact that I've added 2 test products for each site.

Hopefully this information will be useful.