Two Search pages, One search form

Hook into 'template_include' and change the template here.

Made up example, not tested:

add_action( 'template_include', 'wpse_96472_search_template' );

function wpse_96472_search_template( $template )
{
    if ( ! is_search() )
        return $template;

    if ( empty ( $_GET['post_type'] ) )
        return $template;

    if ( 'poster' === $_GET['post_type'] )
        return get_template_directory() . '/poster-search-template.php';

    if ( 'house' === $_GET['post_type'] )
        return get_template_directory() . '/house-search-template.php';

    return $template;
}

You have to change the template names and the $_GET parameter name of course.