Are both copies PS3? The instruction is actually in the PS3 manual/leaflet.

Anyhow the TL;DR is:

Make sure you sign in to your own PS3 user (which has a PSN/SEN account linked) when doing your PS3 Promotion Code redemption.
In XMB > Account Management > Redeem Codes; Enter the 4-4-4 code.
Using your existing FFXIV PS3 Launcher, go through the initial registration.
When you get to the point to select an existing Square-Enix Account or create a new one, MAKE SURE you select the existing (I currently possessed a Square Enix Account) option. Enter your SQEX ID and password. If you don't have One-time toke registered, just leave it blank and log in.

If all goes well, it is activate and your SEN is linked to your SQEX service account. If your PS3 is CE copy, it will ask you which FFXIV service account you want to link to, or option to add a new service account. Make this decision carefully, as different service account means entirely different account (for subscription), use the existing FFXIV service account if you have characters that you want to apply CE items to.

Once all that is done, Play to update the client.

Then to add your brother's copy, make sure you sign out of your PS3 user and switch to his, then repeat the steps above. (To be safe, shut down the PS3 and boot it up and log into his PS3 user.)

Good luck.




P/S: By the way, all the above steps are mentioned in the page 3 of the PS3 manual "Installation Instructions" and the Registration Code also said "Please refer to page 3 of the manual for instructions on how to redeem this code."