not really a hack per-se but if my memory serves me, FF9 on ps1 has natively support for 2 players (decide controller for characters in combat.) i always thought it was pretty cool since it's not really the kind of game you'd think of playing with 2 people.I don't know about FF9, but FF IV definitely does have that option (on PS1). And there are hacks to add this to FF III: http://www.romhacking.net/hacks/1797/ and FF V: http://www.romhacking.net/hacks/2122/.
Nice to see this get some attention. I really love these hacks as well and I will create some myself too. I'll make a personal projects topic once I've made some progress.
BTW: Secret of Evermore is in the NES section.