Hi John!
Here's a few resources that may answer your question.
Different Types of Isolation: Channel-to-Channel, Bank (Channel-to-Bus), and Channel-to-Earth Ground Isolation
http://digital.ni.com/public.nsf/allkb/CF71091043A6193B862573D70077138B
Looking at the manual for the 9264, it appears that the 9264 is not channel-to-channel isolated, and only Channel-To-Earth Ground Isolated. The channel-to-earth ground isolation only protects the channel grounds from the ground on other background processes, such as the power supply. Thus, the 9264 AO channels do share a common ground only to each other.
