install Noah's plugin: https://github.com/NoahY/q2a-network
and follow instructions on https://www.question2answer.org/qa/11956/two-sites-using-same-codebase-and-new-network-sites-plugin
define QA_MYSQL_USERS_PREFIX separately from QA_MYSQL_TABLE_PREFIX. https://www.question2answer.org/qa/11006/how-to-make-two-q2a-s-based-on-one-user-database?show=11936#c11936
use the same values for all the QA_MYSQL_* constants in each site's qa-config.php file
install "sticky sessions" in PHP
(based on Bruno's research at https://www.question2answer.org/qa )