Joining Multiple database problem :(

[eluser]saidai jagan[/eluser]
Hi,
I am new to CI.I have two database.
I call the databases by the following means
$this->db1... and $this->db2...

if I put a query like this (Joining two database in a join query)
$sql = 'SELECT DB1.DBTable1.user_id FROM DB1.DBTable1
LEFT JOIN DB1.DBTable2 on DB1.DBTable2.user_id = DB1.DBTable1.user_id
LEFT JOIN DB2.DBTable1 on DB2.DBTable1.email = DB1.DBTable2.user_email WHERE .......'

and i run the query by,$this->db->query($sql);
it shows the 1142 error. (i.e) SELECT command denied to user 'username'@localhost for table 'DB2.DBTable1'.

Thanks.