double wordpress installation with same database and same posts

One of the nice things about WordPress is the great number of hooks in the code allowing you to extend or override core functionality.

One way to approach this problem would be to set an Apache environment variable in your vhost file for each site that could be used in the WordPress bootstrap process to over-ride the theme and base URL setup.

e.g. in Apache vhost add:

SetEnv WP_CONTEXT main

and

SetEnv WP_CONTEXT mobile

(or equivalent if you’re using a different webserver).

In wp-config.php:

switch ($_SERVER['WP_CONTEXT']) {
    case 'main':
        define('WP_HOME','http://maindomain.com');
        define('WP_SITEURL','http://maindomain.com');
    break;

    case 'mobile':
        define('WP_HOME','http://mobile.maindomain.com');
       define('WP_SITEURL','http://mobile.maindomain.com');
    break;
}

This will set the base URLs based on the environment variable.

Then in plugin add the following filters:

add_filter('template', 'change_theme');
add_filter('option_template', 'change_theme');
add_filter('option_stylesheet', 'change_theme');

function change_theme()
{
    switch ($_SERVER['WP_CONTEXT']) {
        case 'main':
            return 'main';
        break;

        case 'mobile':
           return 'mobile';
        break;
}

This needs to be in a plugin so that it’s loaded before the normal theme loading process (functions.php is part of the theme and hence too late). These filters will intercept and over-ride the theme settings from the database.

The biggest problem with running two sites off the same database with the same content is that all of the links and images in your posts/pages are hard-coded ot the domain that they are uploaded/inserted by. This means that if you insert a link to a page, it goes t othe page on that site only, and it will never link to the same page on the second site.

And again, why do you want two different sites with the same content?