The special meal codes can only be picked from a pre-determined list. Therefore, both diabetic and no-lactose cannot be specified simultaneously. However, the reservation agent can send a put a special note in the PNR for the special request and the reason for it. They can also send a special message to the in-flight catering department regarding this request. I have had occassion where I was able to make request for a very restricted diet meal (not from the list) and able to get it on a multi-leg trip. You might need to call reservations and make sure they understand what your needs are and respond appropriately.