[mitthrowaway2]

If I understand correctly, one reason would be if they have the ability to inspect each others source code (or if they share the same source code), run unit tests, and so on. Basically the same things that humans would do to figure out whether an AI is trustworthy, and which you can't very easily do to a human.

[mitthrowaway2]

Correction: The above is Eliezer Yudkowsky's reasoning. Paul Christiano's is that AIs would cooperate with anyone who would likely be able to gain authority over their reward channel, including other AIs attempting to seize power from humans.