changing databases via $active_group?

I managed to get postgres connected. Here is the connection:

$DB1 = $this->load->database('default', TRUE);
$DB2 = $this->load->database('postgres', TRUE);

My connection to mySQL still works fine without any changes. How do I change the $active_group? I see how to change it in database.php, but I need to change it "on the fly". The image_crud uses $this->ci->db and I'd like it to use $DB2. Is there some way to load a second ci->db structure? 

Also, where did the codeigniter3 documentation go? 

proof that an old dog can learn new tricks