I was able to stack a chromecast code previously, but only once it got to the last month and no money was leftover on the code.
Tried it just now, same situation, 1 month left, but no money on code (as I also get blurays)... and get:
An error occurred during gift redemption - please try again later.
So not so sure they stack, or if they do, it may not be for everyone. I have two chromecast codes left that I'd like to get in before the year is out. Normally it takes 2 months for the code money to be used up for me, so in theory, it should be possible.