If you mean a single copy of the code then it’s not easy but there are ways.
Option 1
If you have access to the server, you can put the code in one folder and create symbolic links. Because the config will be different for each site you’ll need to put index.php and qa-config.php in every folder for each site, then add symbolic links for qa-content, qa-include, qa-lang, qa-plugin, qa-theme.
Option 2
It should be possible to point every site at the same folder and detect which site you’re on in the config, so you can use a different database or table prefix. For example if you’re using subdomains:
if ($_SERVER['HTTP_HOST'] === 'site1.example.com) {
define('QA_MYSQL_TABLE_PREFIX', 'site1_');
} else if ($_SERVER['HTTP_HOST'] === 'site2.example.com) {
//...
}