How does multiple active service accounts work?

[Alhanelem]

If you want to play on the Ps3, there's no need to activate the account you get with the game there (assuming it works like XI did). You just need the game files, then you're good to go with your own accounts.

Incorrect. He either needs to create a seperate account for the PS3, or add the PS3 registration code to his main service account (at which point it cannot be used independently (e.g. u cant log on ps3 and PC at the same time)

You can login with any account on any PC or any PS3, which answers your question, yes you have to select (via username) which account you want to use everytime you launch the game.

You can only og in if that account has a registration code for the platform you're playing on. PS3 players wanting to log on the PC will need to have a PC registration code on their account, and vice versa for PC players. If you want to have two seperate accounts when you buy the other platform's version, they will not be able to freely log on to the opposite platform- they'll only be able to play on the platform coresponding to the registration code that was used.

That's how it was intended to be since the beginning, I've heard nothing of it in a long time so unless someone can point out it changing, I don't expect it to change.