I imported database from my site, and changed it so it can be set up on local host. it worked well. (update: it doesm't work with clean install either)
I installed Q2A and then installed a second Q2a script inside it and set QA_MYSQL_USERS_PREFIX of second instalation as first one's prefix.(I set prefixes inside config file before installing script)
now I went to Users navigation inside sub instalation and there is: "No active users found" message. but when I click on "Special users" and "Blocked users" I see all special and blocked users as it should be(when I asked a question in second site it showed up in user's list).
also when I am loged in with "Admin" user in parrent site I can log out on secund site and log in with a different user.