Oo this thread looks like fun now.
Don't worry about quitting KFC, quit if you have to. I told the managers the same thing, and I'm kinda in a similar position (feeling bad if i leave), but really, they can't force you to stay.
A word of advice though, try and go for PT if you can. I work at a place with maccas next door, and there's an employee who shifted to KFC from there. She was doing casual at maccas, but sometimes they'd give her no shifts, sometimes only a few. It's very unstable. But that really depends on the management, it's up to you whether you choose casual or PT. I think they're very willing for you to do PT, since it means they pay you less. If you want more job security and solid hours, ask for PT at Maccas and KFC. that way, you'll also know that your hours wont clash too (if you're working at both), although at the interview I probably wouldn't tell Maccas you work at KFC since they're competing companies. Maybe try offering half week availability at both, then choose which one you like (or if you'd like to stay at both), and quit the other?
My 2 cents.