Not sure if it helps but I've got two instances of classipress running and both share the same user table so users can log in to either site with the same log in.
As pepsi said the second install is in a sub domain. But the only way it would work was to have the second site URL set up like site2.yoursite.com.
Plus when installing the wordpress part you need to choose a different table prefix but still use the same master database.
For example, both installs would use your master database such as
yoursite_sql, then site 1 would have
wp1_ as its table prefix and site 2 would have
wp2_ (obviously you can choose your own prefixes).
To get the two sites to use the same user table I had to modify the config.php files for each site (
remember to do this before you install wordpress).
Both files would need these details:
PHP Code:
define('DB_NAME', 'yoursite_sql');
/** MySQL database username */
define('DB_USER', 'yourusername');
/** MySQL database password */
define('DB_PASSWORD', 'yourpassword');
/** MySQL hostname */
define('DB_HOST', 'localhost');
/** Database Charset to use in creating database tables. */
define('DB_CHARSET', 'utf8');
/** The Database Collate type. Don't change this if in doubt. */
define('DB_COLLATE', '');
define('CUSTOM_USER_TABLE', 'wp1_users');
define('CUSTOM_USER_META_TABLE', 'wp1_usermeta');
Then lower down the page (around line 65) you will see:
$table_prefix = 'wp_';
Config file 1 would need the prefix for site 1 ie:
$table_prefix = 'wp1_';
and config file 2 would need the prefix for site 2:
$table_prefix = 'wp2_';
Then under
define('WP_DEBUG', false);
You need to add a custom capabilities prefix to site 1:
define('CUSTOM_CAPABILITIES_PREFIX', 'wp1_');
Once done, both config files point each install to the same user table in site 1.
I did all this with one site already installed and modified the config file for site 2 before I installed it.
Finally I also had to make sure the Custom Post Type & Taxonomy URLs were set up differently to the first site in the the classipress advance settings.
I hope this all makes sense, I'm not very good at explanations. And I'm not an expert but I'm sure this would work for Job Roller too.