where by RAW
does it state you can replace a 0 dex/int mod only once, or for what purpose? What if the class ability allowed you to replace your dex mod with your wis mod for druid powers, or something else? According to your logic, that would preclude this. E.g. let's say you use Dilletante and Combat Virtuoso for Dex on a str-based dilletante power. Does that mean Hide Expertise ceases to function? Or that your dex mod is now useless for the rest of your abilities?
They are different sources, one feat vs class ability, so they should stack. They are not from the same named game element. Nothing in the wording of either ability implied they need to be exclusively applied, or in what order.
You are using your Con mod instead of Dex, let's say for AC in Hide. Fine. So your dex or int mod is let's say 0, or 1.
Then, applying the Hide Expertise Feat, get a +2 in exchange for the 0, or 1. Net gain. I don't see what the problem is. It says nothing about Contitution as your AC stat. You aren't losing your Dexterity or Intelligence per se anyway, you are just choosing to apply the feat's benefit for the narrow purposes for which it confers a benefit, and doing the same thing with the class ability.
If it said you can a +2 feat bonus to AC, and the druid Con mod thingy also said you gain the AC as a feat bonus too...fine, they wouldn't stack.
If I'm wrong here, please show a page number reference in a valid rules source that says you cannot swap an ability modifier for one purpose thanks to one feat, and do it again for another feat, and why they shouldn't stack. Logically, they should.
There is no impediment AFAIK to this combo working as the OP said. My point is, even after "swapping" your Dex mod for the +2 or the Con mod, you still have your Dex+Int mods usable fully for other purposes, skill checks, powers, whatever, they are still there. Thus, you can and should be able to redeem it twice.
It's like having two coupons at a store that say : If you own a car, and agree not to drive it today, we will give you 5 dollars. Outside, you meet a hippy who tells you the same thing, but will smoke you up. No big deal, enjoy your buzz AND the 5 bucks, and count your lucky blessings. Not using your car that day. Actually, the wording of those two things are even less restrictive than my coupon example. It should actually be : Agree to not drive any car today, and we'll give you 5 bucks, whether you own a car or not...just don't drive it.
Then you go outside, and hippie says : Hey man, nice bike, since you don't drive a car, let me smoke you. Your Dex/Int is the smokey car you just made up (or did you? heheh), you pretended to have, or didn't because nobody checked, or even cared. All they had were their own requirements for giving you their respective benefits, which you met individually, and never the twain shall meet.
The rules state specifically when similar benefits don't stack. No such wording was used in this case.
Only if the druid ability said you lose your dexterity and mind permanently or until the end of your next turn, and turn into a stone with no dex stat or int stat to speak of and thus disqualify you from fulfilling the feat's requirements, would that prevent Hide Expertise from giving you the +2.
Your con mod bonus to AC is untyped..AFAIK...anyway, it doesn't matter..the Expertise one IS. They stack. The feats don't care what you did with the other feats.
You never *actually* swapped your mods, outside the scope of that feat or ability. They are still there to be swapped ad infinitum.
That said, I expect they will errata Hide Expertise feat in light of this, to read "You can swap your AC ability modifier, whatever that may be, for a flat +2".
Until it says that, or includes Constitution in its list specifically, it doesn't negate the other benefit from applying simultaneously.
Some of you are thinking RAI, but RAW it works. The rules are VERY specific about how things do or do not stack. This is definitely a loophole and I doubt a lot of DMs would allow it, but it works as written.