You need to set QA_MYSQL_USERS_PREFIX on the second installation only.
For example on the first (original) site you might have this
define('QA_MYSQL_TABLE_PREFIX', 'qa_');
And on the second one (sub domain), if it's called "two" you would have this:
define('QA_MYSQL_TABLE_PREFIX', 'qatwo_');
define('QA_MYSQL_USERS_PREFIX', 'qa_');