You can register multiple ffxiv account per SE account.
To dual box like you want, i advise you to use two pc copy, a gamepad and use the checkbox to allow the gamepad when the game dont have focus, this way, you're input will be broadcasted to both ffxiv windows. You will still need a keyboard, rebinds few keys and make few in game macro like /follow /assist, but with only two char, that's not that's hard.