Multiple Database Transactions

How can i do multiple transactions on multiple databases?

$transactionManager = new TransactionManager();
$transactionManager->setDbService("db1");
$db1_tx = $transactionManager->get();

$transactionManager = new TransactionManager();
$transactionManager->setDbService("db2");
$db2_tx = $transactionManager->get();

$obj = new Obj();
$obj->setTransaction($db1_tx);

if(!$obj->save($_POST)){
    $db1_tx->rollback();
}else{
    $obj2 =  new Obj2();
    $obj2->setTransaction($db2_tx); //since it belongs to db2
    if(!obj2->save($_POST)){
        $db2_tx->rollback();
        $db1_tx->rollback();
    }
}

$db1_tx->commit();
$db2_tx->commit();

The above code does not work, it only hangs my mysql server