Thanks everyone for the help.

I have, in theory only one downloaded version in the PS3. But I suppose that changing PSN and login with a SE account linked to that PSN would let me use the same downloaded data of the game, working like this:

Using the management account nº1 of the PS3 -> Using my PSN account -> FFXIV game -> Using my SE account.
Using the management account nº1 of the PS3 -> Using her PSN account -> FFXIV game -> Using her SE account.

Basically, what we have downloaded in the PS3 is the game, the launcher, but I suppose I don't need to buy a second one if we already have our different PSN and SE accounts linked and with their time periods. (For example she has around 29 days remaining with her account and I around 12).

I'll take a look to it later and confirm it.

Even so, thank you for solving my main fears about couldn't be back to log in PC and such!