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

[Daeriion_Aeradiir]

You can have multiple instances of FF14 running on the same machine. I believe the limit is 2 simultaneously, but that might have changed to 3 at some point, I can't confirm. Either way, the launcher will refuse to start/give a warning once you've hit that limit so you can open 2 instances and see if it'll let you try for a 3rd to get your answer. Once an instance is opened, you just change it to windowed, move it to whichever monitor you want it to be on, then change it to borderless windowed to snap it to that monitor.

I cannot confirm or deny if multiple service accounts on the same Square Enix account allows simultaneous play - but the #1 in your conclusions is exactly how one of my friends plays with multiple accounts, by having two entirely separate Square Enix account, each having bought FF14, EW expansion, each with a mogstation subscription, etc. At the very least, I can guarantee that works.

However, there are some cumbersome things regarding using multiple instances on the same machine:

-System configuration is saved locally on a client exit, meaning next time the clients load up, they'll both be using the system configuration from the last client closed. This means if you want to change graphic settings for one client and not the other, you'd have to repeat it every single time you boot them up. This can also be problematic if your monitors are of different resolutions, since it'll cause HUD elements to be more squished/expanded depending. Effectively, you'd have to figure out a universal settings list for your system configuration you'd be fine using for all clients involved.
-If your purpose is to have a family member/a friend/significant other/etc playing with you simultaneously, I think controls will be a huge issue, since I don't believe FF14 has the ability to detect multiple mice/keyboards and individually assign them to separate clients. But I have no idea on this front, so you'd have to look into this if that is your intention / someone can confirm/deny for me. If you just want to have two accounts open and alt tab between them, this isn't a problem.