No utilice la sintaxis de combinación implícita. Se necesita un segundo conjunto de uniones para obtener un segundo atributo.
SELECT upa1.value, upa2.value, up1.status, u1.status
FROM product p
/* These joins get attribute/value for 'username' attribute */
INNER JOIN product_attribute pa1
ON p.product_id = pa1.product_id
AND pa1.attribute_name = 'username'
INNER JOIN user_product_attribute upa1
ON pa1.product_attribute_id = upa1.product_attribute_id
INNER JOIN user_product up1
ON upa1.user_product_id = up1.user_product_id
INNER JOIN user u1
ON up1.user_id = u1.user_id
/* These joins get attribute/value for 'domain' attribute */
INNER JOIN user_product_attribute upa2
ON upa1.user_product_id = upa2.user_product_id
INNER JOIN product_attribute pa2
ON upa2.product_attribute_id = pa2.product_attribute_id
AND pa2.attribute_name = 'domain'
WHERE p.product_name = 'email'
