I've done some serious mult-boxing during my school years, for lineage 2 and Anarchy Online (when it went f2p)
In lineage 2 i had 6 accounts and i set it up so that hitting one button would do something different for each character. and i would have healing buttons on my hotbar for healers, that would otherwise be empty for the dps and tank. And i would have a /follow button for the DPS and Healers that would be empty on the account that was on my main window of focus.
This was especially easy for lineage 2 because about 55% of melee damage came from auto-attacks.
I didnt do it for farming, i did it to play solo in Anarchy Online because leveling up players were extremely scarce.
Lineage II i did it to be a force to be reckoned with, scare off those 1-4 PK players from attacking a party of 1 tank, 4 daggers and 2 healers. When they did have the courage to attack, my force was swift in killing, having 6 characters focus target is a beautiful thing.
Example:
keystroke 1 (Attack)
Tank = /attack
DPS = /assist <tankname> + /attack
Healer = /assist <tankname> + /attack (or /healer nuke)
keystroke 2 (target tank + follow)
Tank = empty
DPS = /target <tankname> + /follow
Healer = /target <tankname> + /follow
keystroke 3 (special attack 1)
Tank = /special attack1
DPS = /special attack1
Healer = /special attack 1
keystroke 4 ( special attack 2)
" "
keystroke 5 ( Target tank and heal)
Tank = /special attack 1 & 2
DPS = /special attack 1 & 2
Healer = /target <tankname> + /heal + /previous target/
keystroke 6 (AoE heal)
Tank = /special attack 1 & 2
DPS = /special attack 1 & 2
Healer = /aoeheal
I've been reported in both Lineage II and Anarchy Online and when i explained how it worked they were like "oh i guess that's not botting, you are free to go."
Mind you this was over 8 years ago bots have come a long way since then, but for the most part still easy to identify because they cant make the same sound judgements as a human.
As for FFXIV, having a similar keystroke/multibox set up would be difficult because for maximum damage every class has a varying rotation that cant be matched by creative keystroke setups. Having a heal bot (when i say bot i mean dualboxing jargon) follow you around might work though, but a chocobo companion fills that role very well.