Allora per il momento ho risolto impostando la query in questo modo:
[PHP]
SELECT accounts.id, accounts.name, accounts.billing_address_street, accounts.billing_address_city, accounts.billing_address_state, accounts.billing_address_postalcode, accounts.billing_address_country, accounts.rating, accounts.phone_office, accounts.phone_alternate, accounts_cstm.id_anagrafica_c, accounts_cstm.tipo_pagamento_c, accounts_cstm.data_firma_c, accounts_cstm.quota_associativa_c, accounts_cstm.data_incasso_c, accounts_cstm.piva_c, accounts_cstm.codice_fiscale_c, accounts_cstm.stato_adesione_c, accounts_cstm.data_invio_kit_c, accounts_cstm.stato_kit_associato_c
FROM accounts
INNER JOIN accounts_cstm ON accounts.id = accounts_cstm.id_c
[/PHP]
Grazie per la disponibilità