Well, a narrow reading of the rules would say they stack. Technically.
However, looking more closely, they are very, very similar bonuses. Each goes away when you lose the Dex bonus to AC.
I'd rule they stack, but only in the sense that levels from both prestige classes add up together for the bonus, not that you get the bonus twice.
What books are these classes from? If from the same book, I MIGHT be inclined to think that the differecne between these two abilities was deliberate and they add together, if from seperate books I'd be MUCH less generous because it's a little too easy to create abuses in powergaming by combining material from various supplements in unanticipated ways.