内部参加のカウントとすべてのアイテムのカウントを取得するにはどうすればよいですか？

2つのクエリをユニオン互換にすると、結果を組み合わせて、追加のクエリで集計できます。

    SELECT payer, 
           sum(churned_accounts) AS "churned_count",
           sum(total_accounts) AS "total_count" 
    FROM (
      SELECT CAST(payor_id AS CHAR(50)) AS "payer", 
             count(*) as "churned accounts", 
             0 AS "total accounts" 
      FROM paid_users_no_more 
      INNER JOIN paid_users 
        ON paid_users_no_more.user_id=paid_users.user_id 
      WHERE paid_users.payment_start_date NOT BETWEEN '2015-08-01 00:00:00'::timestamp AND '2015-08-30 23:59:59'::timestamp 
      AND paid_users_no_more.payment_stop_date BETWEEN '2015-08-01 00:00:00'::timestamp AND '2015-08-30 23:59:59'::timestamp 
      GROUP BY paid_users.payor_id

      UNION

      SELECT CAST(paid_users.payor_email AS CHAR(50)) AS "payer", 
             0 AS "churned accounts", 
             count(*) AS "total accounts" 
      FROM paid_users 
      WHERE paid_users.payment_start_date NOT BETWEEN '2015-08-01 00:00:00'::timestamp AND '2015-08-30 23:59:59'::timestamp 
      GROUP BY paid_users.payor_email
    ) as All_Accounts

0 AS "total_accounts"を持つ および0 AS "churned_accounts" 2つのクエリに同じフィールドが存在することを意味します。これによりUNION 可能です。