It's possible to have the same domain and several web instances, it depends on configuration, also you can make UNA to work on any domain as well if you need, to do this just change:

define('BX_DOL_URL_ROOT', 'https://example.com/');

to:

define('BX_DOL_URL_ROOT', ((isset($_SERVER['HTTPS']) && $_SERVER['HTTPS'] == 'on') || (!empty($_SERVER['HTTP_X_FORWARDED_PROTO']) && $_SERVER['HTTP_X_FORWARDED_PROTO'] == 'https' || !empty($_SERVER['HTTP_X_FORWARDED_SSL']) && $_SERVER['HTTP_X_FORWARDED_SSL'] == 'on') ? 'https' : 'http') . '://' . (isset($_SERVER['HTTP_HOST']) ? $_SERVER['HTTP_HOST'] : 'https://example.com/') . '/');