If applying both codes simultaneously won't give 60 days, I doubt applying them a month or more apart would, either.
We know that SE is including 30 days free trial for registering the PS3 code to an existing v1 service account. To me, the simplest explanation is that the 30 days free comes with each new platform registration per service account. By that policy, a new customer registering both codes at launch should get 60 days free trial, but SE might have decided to limit free trials to one per service account. We won't know until SE clarifies their policy or when new customers apply both PS3 and PC codes to their account at launch.