/* Only Show 2 Category in Single Post */
function show_only_2_category( $thelist ) {
	if ( is_single() && is_singular() ) {
		$thelist = array( $thelist[0], $thelist[1] ); /* Set Only First 2 Category */
		return $thelist;
	}

	return $thelist;
}
add_filter( 'the_category_list', 'show_only_2_category', 999 );