Опитайте това:
SELECT `customer`.*, `ca1`.`value1` AS `wedding_date`, `ca2`.`value1` AS `test`
FROM `customer`
LEFT JOIN `customer_attributes` AS `ca1` ON customer.customerID = ca1.customerID AND ca1.key1='wedding_date'
LEFT JOIN `customer_attributes` AS `ca2` ON customer.customerID = ca2.customerID AND ca2.key1='test'
WHERE (customer.customerID = '58029')
Преместването на условията 2 WHERE на ca1/ca2 в условието JOIN вместо това трябва да го сортира