1st answer: You get Hinnah and Hannah by completing the guild task 70. You then find her in one of the guilds.
2nd answer: To be exact, it's the guild task 70 of the Union of the Golden Chalice guild. The objective is "Defeat the The Enlightened Seven", which is DLC on 360, on PC no DLC is needed as it is included in the retail game. First of all you need to be guild rank 5 in above named guild by doing several tasks. Then you need the Eye of Promise, which you get on the 360 just by downloading the DLC, on the PC you have to do guild task 12 of the Union of the Golden Chalice guild first which is slaying 20 Weredragons. The fight itself is against the Enlightened Seven, which will be all present on one battlefield (not all at once, but 2 at times). EDIT: Was too sure about that...I did all the quests I mentioned above but I can't hire Hannah and Hinnah yet, others like Young, Milton, Ludope and Zuide are ready for hire...hmmm anyone knows the missing puzzle part?
- Remember there is a Quest that HANNAH gives you in BALTERROSA's pub. you have to look for HINNAH in the Mojcado Castle. perhaps you didnt do this one?. (edit, just read the wikianswers, and yes, its a REQUIREMENT to do the "blooming flower, singing bird" quest.