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https://fishpig.co.uk/magento/wordpress-integration/layout-xml/

1) open app/design/frontend/base/default/layout/wordpress.xml

2) add in wordpress_post_view  a reference for the single posts template:

<wordpress_post_view>
    .
    .
    .
		<reference name="root">
			<action method="setTemplate">
				<template>page/2columns-left.phtml</template>
			</action>
		</reference>
		.
		.
		.
		
</wordpress_post_view>