How to use set_query_vars

Hi,

I’m trying to make a plugin that filters posts that appear on 2 categories…

For testing purposes I did this:

add_action('parse_request', 'dualcatsfilter' );

function dualcatsfilter( $cat1 = 14, $cat2 = 79 ){
	global $wp;
	$wp->set_query_var('category__and', array($cat1, $cat2));
}

But it doesn’t work… I’m thinking that maybe I didn’t understand how to use the set_query_var function.

Can anyone help me around?

Thanks