[Casey6]

Well from the way the manga has been going, I am not sure we will find out if they were working together in the beginning.

But I do think they were and Aizen changed on Urahara.

Urahara is known for working with the insain - ha

Because I am just guessing, all of us are just guessing. there is no way for us to find out unless they write something about it.

I really believe that Urahara and Aizen were working together in the beginning. I think that Urahara believed that working with the insain he could change them, make them productive, just like he gave Mayuri a chance.

I think he knew that Aizen was mad, and he was working with him to try and test his results, but since it was a disaster and Aizen proved to be more evil and more insain then the others he had worked with in the past, and in the end he (urahara) was left blamed for the horror they created.