Okay I'm quoting this from the US Amazon Heavensward Digital CE page:

Both Early Access and DLC will be run through one combined code that will be emailed before the Early Access period begins. Once customers receive the codes, they will have to go to Square’s dedicated website, select their platform, and another code will be generated. They then take this other code and enter it into either PSN (PS3/PS4) or MogStation (PC).

The MogStation took the first code they gave me and generated no additional code, so this sounds misleading. There is no mention of two codes at the start -- one for the expansion, and then another for Early Access + DLC.