how to check table data with another db config?

[eluser]kreamik[/eluser]
if i have one db config with
hostname,username,password,database = localhost,user1,pass1,virtual1

then i need to check another table if exist, what should i do?

Code:

$this->load->database();
$check = $this->db->table_exists('username'); to check table 'username' in current database

what should i do if i need to check table username in virtual2 db with only one db config?

i test with:

Code:

$check = $this->db->table_exists('virtual2.username');

and it always give me no result;