Ok, I've done a lot of searching around and have found many answers to this, some contradicting each other So I will ask it here and hope I get a clear answer.

Here is the situation:
I play FFXIV on the PS4, my wife will be starting soon, on the PC. There are some days when I wish I could play on the PC (as to not hog the television in the living room) an I know you have to buy a separate licence for PC. We plan to pay $30 for the pc licence and tie the key to her account.

Here is my question:
Will I have to purchase a third copy of the game for me to play on PC as well (as stated above, the PC key will be tied to her account) and if so, will I have to have 2 PC versions installed on the same machine? Or is this all much more simple, and I can just use my credentials on the login screen and play on the PC through her copy. (also I believe as they are on the same Wireless router, the PC and PS4 should be on the same IP address.)

Thanks in advance!