Search for single post by 2 tags

Using the tax_query param in your args (and replacing the dot with a comma) should get you what you’re looking for:

$args = array(
    'tax_query' => array(
        'relation' => 'AND',
        array(
            'taxonomy' => 'post_tag',
            'field'    => 'slug',
            'terms'    => 'fetch',
        ),
        array(
            'taxonomy' => 'post_tag',
            'field'    => 'slug',
            'terms'    => 'agirt',
        )
    )
);