$args = array(
'post_type' => 'product',
'nopaging' => true,
'tax_query' => array(
'relation' => 'OR',
array(
'taxonomy' => 'product_cat',
'field' => 'id',
'terms' => 53,
),
array(
'taxonomy' => 'product_cat',
'field' => 'id',
'terms' => 51,
)
)
);