Start two different session

How should I do if I wanted to be able to start two different sessions? One for the site, and then for users and a just for acp?

Why do you need two different sessions? Just use two different variables, one for the site and one for anything else, and unset them when they are not required.

Yes yes I realized I got there alone.

I just change
$this->session->set_userdata('"this"', $sess_array);