Registering for PC and PS3

[Demeos]

Yes, you'll need a copy of both per account. One pc and ps3 version for yourself and One pc and ps3 version for the other.

You: 1 copy of both the PC and PS3 version.
Other: 1 copy of both the PC and PS3 version.

So if you and one other person wish to play together in FFXIV on either system of your choice, you'll need 2 copies of the PS3 version and 2 copies of the PC version total.

...Regardless if you have two seperate Square-Enix accounts or one Square-Enix account with two service accounts, you'll still need that many copies between yourself and one other as each copy when activated is assigned to a specific service account.