Considering each head has it's own personality, I would think so. I assume this isn't the case for most multi-headed monsters however, as most don't have multiple personalities.
No. Mind-affecting and soul taking magic functions normally on them so having multiple brains doesn't help them in this regard.
A human with multiple personality disorder doesn't have multiple souls (IMO I suppose); an Ettin won't have 2 souls.
Well we have things like MASTER CHYMIST in Pathfinder, and they don't say anything abiut having two souls so I think you're correct.
Yes.
Suppose you reincarnated an ettin; would you get an elf with MPD or an elf and a gnome (requiring 2 spells, probably)? I'd argue the latter, seeing an ettin as more like siamese twins than like a single person with a mental disorder. There are two actual brains there.
Pathfinder basically defines souls as just an autonomous collection of positive energy. It's not a personality or a consciousness, although it may be judged based on the decisions of those things.
So, in PF, an ettin has two brains, two personalities, and probably two consciousnesses, but just one lump of positive energy.
Great question, actually.
ogres (and humans with racial heritage(ogre) ) with this feat might very well have two souls. as the other head takes on some of the will-save requiring effects on them instead of the main head (chance Depand if taking one or two feats, but the fact there is one seem to indicate a separate being)
If they only have one soul, do they still have two minds when they become a petitioner? Or are the two personalities combined somehow?
If they only have one soul, do they still have two minds when they become a petitioner? Or are the two personalities combined somehow?
It's possible that a petitioner's mind or personality is "turned off" or suppressed, somehow. I mean, I know this is fantasy, so it's all made up, but I'm not sure if Pathfinder's canon actually details it this much, so it's left to us to make up. But I imagine the line to be judged is a timeless, drab, dismal place where these souls (whatever they are) are just waiting around to be judged. During this time, I can see strong personalities being suppressed so as to not incite trouble, and maybe even all personalities if a soul can get impatient or bored or whatever. If they're suppressed, why not just turned off? I don't think they're wiped, because of resurrection spells, but I'm not sure that things like minds and personalities would stay intact in the same way as when a soul was alive.
The Ettin just has multiple personalities, but remains one creature.
The Ceratiodi, on the other hand, are indeed two separate entities sharing the same body.
Maybe? But I think it's a mistake to automatically equate a Pathfinder "soul" with a mind or even assuming a soul and an entity have a 1:1 ratio. Many real-life traditions do that, true, but I can't find anything Paizo has said about souls other than what I linked above.
I'm contending this because I'm trying to come up with a solution based on what Paizo has said about souls (and any mechanical effects that may be assumed) that isn't contradicted other things Paizo has said. A soul just being a mass of positive energy separate from anything resembling a consciousness seems to work.