Ok, I just unspec'ed my lock and tried this.
After some testing this is what I came up with...
On my screen it does appear than you can cast Coa while casting Corruption.
As soon as the first GCD is over for the initial corruption cast you can cast Coa and start its GCD (before the corruption is done casting).
So it LOOKS like on my screen that I am starting the Coa GCD .5 seconds before corruption is finished. Meaning I only have to wait half as long before my next cast.
BUT, I believe this may be associated with the new GCD system and the way it accounts for server lag. I only have a .1 latency though supposedly. So I can't explain why it would let me start the GCD for Coa so early.
Screenshot: As you can see by the red text it looks like (on my client) that all three were applied within the same second without having to wait for another GCD. But if you look at the white timestamp text from the server, it says I started casting them well within GCD limits.
Conclusion: New GCD system accounting for server/client lag. Easily mistaken for not having to wait for a GCD for someone who has more latency than me.