So, by the RAW, dodge bonuses always stack. So the +6 is the correct answer.
However, @Steve G points to an indecision on the Paizo forums about this very rule. And I personally agree that this can become very abusive. At level 8 with two-weapon fighting, I'm rolling 4d6 sneak attacks and I can reasonably get 3 or 4 hits, which amounts to a +12AC bonus.
Again, @Steve G points to some possible solutions, but I think they're both sub-par. Tracking the "creature hit" is a real pain, especially in a big fight. And good rogues are sneak attacking all of the time, so this becomes a constant thing to remember.
The "does not stack with itself" is better, but I'm not sure how to convey this cleanly. I think the best way to rule this is simply to add a max bonus. I would say, take your pick on that number, see which one works best.
- Max = sneak attack damage dice
- Max = rogue levels