I have 3 joomla templates made by artisteer , I override com_k2/templates/X/category_item.php on each templates' html folder,
now , I cant access to those overrides choices , on K2-category page it shows only the overrides under the default joomla template and ignore those under the left two joomla templates .

Make sure the k2 templates are in www-root/templates/the-artisteer-template-name/html/com_k2/your-k2-template-name/category_item.php

Then they will certainly show up in the category settings.

This will come in handy as well getk2.org/documentation/tutorials/174-templating-with-k2-and-the-concepts-of-sub-templates