Thanks for the link. It is a very interesting read.
In particular it also says this:
To avoid both signals being added by the system, a priority P/B is provided on each stick. By pressing this button, a pilot may cancel the inputs of the other pilot.
Yes, indeed, I have not found any reliable source for the claim that both pilots were making significant stick inputs simultaneously for any extended period of time.
Do you have a url/link/pdf that describes this behaviour?