[EC-CUBE] EC-CUBE3.0.15
[レンタルサーバ] エックスサーバー
[PHP] 5.6
[データベース] mysql5.7

[現象]
下記サイトにあるように店舗A、店舗Bの会員情報を共通化しました。
https://qiita.com/chihiro-adachi/items/bb47fa419bb13e2c339a

ただ、上記設定後
会員ログインした後に商品を購入しようとすると下記エラーがでます。

ForeignKeyConstraintViolationException in AbstractMySQLDriver.php line 60:
＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝

An exception occurred while executing 'INSERT INTO dtb_order (pre_order_id, message, order_name01, order_name02, order_kana01, order_kana02, order_company_name, order_email, order_tel01, order_tel02, order_tel03, order_fax01, order_fax02, order_fax03, order_zip01, order_zip02, order_zipcode, order_addr01, order_addr02, order_birth, subtotal, discount, delivery_fee_total, charge, tax, total, payment_total, payment_method, note, create_date, update_date, order_date, commit_date, payment_date, del_flg, customer_id, order_country_id, order_pref, order_sex, order_job, payment_id, device_type_id, status) VALUES (?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?)' with params ["049473d47d7bdd4d4b0c53c628d93e6b4ac1bebc", null, "\u4ef2\u91ce", "\u7fd4\u592a", "\u30ca\u30ab\u30ce", "\u30b7\u30e7\u30a6\u30bf", null, "info@example.com";, "048", "000", "0000", null, null, null, "335", "0004", "3350004", "\u8568\u5e02\u4e2d\u592e", "3-19-20", "1993-06-06 00:00:00", 8100, 0, 1000, "0", 0, 9100, 9100, "\u4ee3\u91d1\u5f15\u63db", null, "2018-05-25 10:28:54", "2018-05-25 10:28:54", null, null, null, 0, 3465, null, "11", "1", "18", 4, 10, 8]:

SQLSTATE[23000]: Integrity constraint violation: 1452 Cannot add or update a child row: a foreign key constraint fails (`店舗Bのデータベース`.`dtb_order`, CONSTRAINT `FK_1333XXXXXXXXXX` FOREIGN KEY (`customer_id`) REFERENCES `dtb_customer` (`customer_id`))

＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝

仮に購入情報を店舗Aのデータベースに入れるとエラーは無くなります。
ただその際店舗Aにしか購入情報が入りません。

上手く購入情報を店舗A、店舗B両方に入ると良いのですが、