How to insert 2 args into 1 Wp_Query for a slideshow

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You could try merging the two arg arrays:

$args = array(
    'post_type' => array('post','page'),
    'tax_query' => array(
       'relation' => 'OR',
       array(
         'taxonomy' => 'category',
         'field' => 'slug',
         'terms' => 'slider'
      ),
      array(
         'taxonomy' => 'my_taxonomy',
         'field' => 'slug',
         'terms' => 'slider'
      ),
    ),
  );