At this I don't think you can merge accounts. (Please someone correct me if I am wrong). Normally a person has one account for the duration of the game. Now if you want multi platform then what you do is add the new platform codes to the current account. So for example, I am a PS3 user account. I bought the PC verison and when it asked if I have a Square Enix account I would put Yes and then that PC code would be applied to the account. I myself currently have a PS3/PS4/PC account after buying a copy of each. So I can log in with any device.