drush

— AFAIK, ashtei occurs only in conjunction with "asar" and it adds ONE (not two) to the asar, making eleven (not twelve). It is one of the sources for the notion that kol hamosif goreia; shtei-asar if we ignore the mixed gender would mean twelve; adding an ayin to it reduces it to eleven. Admittedly, that is more in the realm of drush than formal linguistics.