così dovrebbe funzionare... prova.Codice PHP:
$host="localhost";
$db="db_1";
$user="root";
$pass="pw";
$link1 = mysqli_connect( $host, $user, $pass, $db )
or die("Impossibile connettersi al Database - $db");
$host2="localhost";
$db2="db_2";
$user2="root";
$pass2="pw";
$link2 = mysqli_connect( $host2, $user2, $pass2, $db2 )
or die("Impossibile connettersi al Database - $db2");