query.php

<?php
$args = array(
  'post_type' => 'post',
  'tax_query' => array(),
);
if ( $filtercategory ) {
  $args['tax_query'][] = array(
    'taxonomy' => 'category',
    'terms' => $filtercategory,
    'field' => 'term_id',
  );
}
if ( $filtertag ) {
  $args['tax_query'][] = array(
    'taxonomy' => 'post_tag',
    'terms' => $filtertag,
    'field' => 'term_id',
  );
}
if ( $filtercategory  && $filtertag ) {
  $args['tax_query']['relation'] = 'AND';
}