Can I play multiple simultaneous instances of the game on the SAME Windows PC?

[Canadane]

You can definitely play on 2 instances using a single Square Enix account with 2 FFXIV service accounts. First, launch FFXIV, choose your datacenter, then it will ask which service account you want to use. Pick a service account, then login with your character is usual. Then open up the second window. Choose a datacenter, select your second service account, and pick a character. You now have both accounts logged in. After both instances of FFXIV are logged in to their respective service accounts, you can freely change characters within the same datacenter by logging out and then just clicking start. Each FFXIV window will remember which service account it was logged in to

Hey even I learned something today, neat!
Though seems functionally the same as two separate SE accounts but you only have one login credential to remember.