How to do selected single product should be purchase alone?

app->code->community->Brst->Test->etc->config.xml

<?xml version="1.0"?>
<config>
    <modules>
        <Brst_Test>
            <version>1.0.0.0</version>
        </Brst_Test>
    </modules>
    <global>
        <models>
            <brst_test>
                <class>Brst_Test_Model</class>
            </brst_test>
        </models>
    </global>
    <frontend>
        <events>
            <controller_action_predispatch_checkout_cart_add>
                <observers>
                    <Brst_Test_log_cart_add>
                        <class>Brst_Test/observer</class>
                        <method>logCartAdd</method>
                    </Brst_Test_log_cart_add>
                </observers>
            </controller_action_predispatch_checkout_cart_add>
        </events>
    </frontend>
</config>

app->code->community->Brst->Test->Model->Observer.php

<?php
Mage::log('Hy observer called', null, 'logfile.log');
class Brst_Test_Model_Observer
{
    //Put any event as per your requirement
    public function logCartAdd() {
        $product = Mage::getModel('catalog/product')
                        ->load(Mage::app()->getRequest()->getParam('product', 0));
        $cart_qty = (int) Mage::getModel('checkout/cart')->getQuote()->getItemsQty();

        if ($product->getId()==31588 && cart_qty > 0) {
            Mage::throwException("You can not add This special Product, empty cart before add it");
        }

        // $quote = Mage::getSingleton('checkout/session')->getQuote();
        // if ($quote->hasProductId(2)) {
        //  Mage::getSingleton("core/session")->addError("Cart has Special Product you can not add another");
        //  return;
        // }
        $quote = Mage::getModel('checkout/cart')->getQuote();
        foreach ($quote->getAllItems() as $item) {
            $productId = $item->getProductId();
            if($productId==2){
                Mage::throwException("Cart has Special Product you can not add another");
            }
        }

    }
}
?>

app->etc->modules->Brst_Test.xml

<?xml version="1.0"?>
<config>
	<modules>
		<Brst_Test>
			<active>true</active>
			<codePool>community</codePool>
		</Brst_Test>
	</modules>
</config>