Posts modified in the last 48 hours

You can do it easily with WP_Query:

$args = array(
    'date_query' => array(
        'relation'   => 'OR',
        array(
            'column'  => 'post_date',
            'after'   => '-2 days'
        ),
        array(
            'column'  => 'post_modified',
            'after'   => '-2 days'
        )
    )
);

$query = new WP_Query( $args );

More information about date_query parameter of WP_Query.

If, for some reason, you still want to alter the WHERE clausele, something like this should work (not tested):

function filter_where($where="") {
    $where .= " AND ( post_date > '" . date('Y-m-d', strtotime('-2 days')) . " OR post_modified > " . date('Y-m-d', strtotime('-2 days')) . " )'";
    return $where;
  }
add_filter('posts_where', 'filter_where');

Also, you must note that in the post where filter, you are using Y-m-d date format, which doesn’t include time. That can cause unexpected results; you may need to use MySQL datetime format: Y-m-d H:i:s.