"Common knowledge" is a highly misleading piece of technical terminology - in the context of Aumann's paper.
A two-person Aumann agreement exchange example (of degrees C warming next century) looks like:
A: I think it's 1.0 degrees...
B: Well, I think it's 2.0 degrees...
A: Well, in that case, I think it's 1.2 degrees...
B: Well, in that case, I think it's 1.99 degrees...
A: Well...
The information exchanged is not remotely like all the pertinent knowledge - and so making the exchange is often relatively quick and easy.
That is not the definition at the top of the paper you just linked for me: http://www.ma.huji.ac.il/~raumann/pdf/Agreeing%20to%20Disagree.pdf
"Two people, 1 and 2, are said to have common knowledge of an event E if both know it, 1 knows that 2 knows it, 2 knows that 1 knows is, 1 knows that 2 knows that 1 knows it, and so on."
Or I may have missed you point entirely. You have introduced the concept of "Aumann agreement exchange" but not what misconception you are trying to clear up with it.
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