I talked via netflix chat to many CSR regarding subscription codes, I got confirmation from all of them that they have upgraded their system to take codes and stack them one on top of the other.
So if you have 6 month left, and you apply 3 month code, you will have 9 month in total. you can apply as many code as you want and they will all stack.
Please be kind with my opened thread.
Edit: when you apply the code, the billing will not update for some reason, although the code was taken, contact netflix (call or chat) they'll confirm you have extended your subscription
Edit 2: sorry if I misspoke, you can't add chromecast codes to the same account since the terms and conditions are 1 per account, however you can add all other codes from other promotions and they will all stack
Edit 3: Now SDers are reporting that we can stack as many chromecast codes as we want, needs to be confirmed, we will follow closely lol