Filter out a meta key in the Search results page with two CPT

Question

You could add a second array to check if the ‘DTEND’ value is not set:

    $args = array_merge( 
        $wp_query->query_vars, 
        array(
            'meta_query'=> array( 
                'relation' => 'OR',
                array(
                     'key' => 'DTEND',
                     'compare' => '>=',
                     'value' => $currentdate,
                ),
                array(
                     'key' => 'DTEND',
                     'compare' => 'NOT EXISTS',
                )
            ),
        )
    );

That way, any custom posts without a DTEND meta value will also be allowed.