CodeIgniter Sql

[eluser]Unknown[/eluser]
Hello I have a question with the sql with codeigniter.

I want to do this type of things :
( it is not syntaxe sql I know )
WHERE ( a = b AND c = d ) OR ( a = d AND c = b )

For do this I do :
$array = array( a => b, c => d);
$array2 = array(a => d, c => b);

// the rest of my code is ok with the database.
$db_default->where($array);
$db_default->or_where($array2);

When I do that the first part of array2 is ignored ( a => d ) and all the rest work.

Do you have a idea?