Solution
There isn't a specification on the isolation or resistance between the modules. The information about the cRIO's isolation voltage as well as the channel-to-channel and channel-to-earth ground specifications of a module should be used as a guidance instead.
This way, the safety and isolation limits will be determined entirely by the modules and how the external voltage transformers scale and isolate the signals before they reach the module inputs.
As long as the input signals remain within the module's specified limits, there's no risk of overvoltage damage within the chassis.