Umm... Well, I personally think that the Ashura from Shurano and Fai's Ashura are the same person in so far as the share the same soul/personality/core; they are different versions of the same person, as we've seen countless times before in Tsubasa. I suppose it's possible that they're not- that one is the other's father or whatever, but I really don't see any evidence to support that.
Read this bit from Chapter 72:
Kurogane: You still haven't met that someone from your world, have you? The reason you keep running away. It's the same face... However, it's not necessarily the same person.
Fai (with the single most angsty look he's EVER HAD):I would know... if it's just the same face or if it's the same person. I would know.
Another translation of same bit:
Kurogane: Well, you haven't met yours, either, have you? Your little reason why you're on the run. But if he's got the same face... You might not be able to tell.
Fai (with the single most angsty look he's ever had- I really can't express just how angsty and hot it is): I'd know. I'd know if he just looked the same or if it were really him. I would know.
This entire conversation is started because they've just met Piffle Country's version of Tomoyo. Fai had just commented how that Tomoyo looked exactly like Kuro's Tomoyo but wasn't actually the same person. The whole sharing-the-same-soul-but-living completely-different-lives thing.
Kuro comments back about how he (Fai) hadn't met the person he's running away from (Ashura) either. He is almost certainly refering to the version of Ashura they met in Shurano and the fact that he looked exactly like the Ashura Fai had sealed away. *IF* the Shurano Ashura and Ashura that Fai sealed are not different versions of each other, how much sense does this conversation make?
Besides, to have TWO different souls with the name Ashura would confuse things far, far far too much, even for Clamp. Clamp loves to pull weird crap, but that would just be unnecessarily complicated, as if Tsubasa weren't complicated enough with all its different versions of the same soul. No, I definitely think that they are simply two versions of the same person. If not, that would absolutely over-complicate things. There is such a thing as too much complexity, and that would, I think cross the line. It would serve no purpose whatsoever that I can think of.
Somebody earlier mentioned about Fai's Ashura and Shurano's Ashura having different hairstyles- well, Piffle-World Tomoyo has a completely different hairstyle from Kuro-Tomoyo. Hairstyle by itself is not a reliable way to distinguish between characters.
Somebody also said earlier that Fai wasn't scared of Ashura... Well, you can certainly interpret things however you want, but personally, if Fai weren't scared of Ashura, why exactly is Fai running from him? Because he's scared for Ashura, trying to protect Ashura? That has some validity, but I'm far from convinced.
My pet theory is that Fai and Ashura were lovers, along with Fai being his wizard. Ashura, however, did something or another, or a great deal or something or anothers, and Fai eventually was forced to betray and seal him away to prevent Ashura from inflicting any more evil on the world. Thus, why Fai is so angsty and depressed- he was forced to betray the man/woman/whatever he had deeply loved and respected.
And I don't think that Fai was a prince or anything. First, Chii calling Fai 'King'? That was a mistranslation (thanks a lot, DelRay). Plus, you know how the beginning of each chapter starts with a sketch of a character(s) along with a little catchphrase? Well, I'm ninety percent certain that one had a picture of Fai, and said mentioned something about 'sealing his master.' Don't quote me on that, but I'm pretty sure.
I too believe that Fai and Kuro haven't told the whole truth about Shurano. If they couldn't talk, then how did they come up with their plan to pretend not to know Syaoran on the battlefield? They must, at some point, have been able to communicate. Whether they talked about Ashura or not, I don't know. Maybe.