Neos companion cases - prompt me to tag or save in both cases on front end

If a case is in a companion case group, I want to be prompted on the front end every time I create a document, or tag an email, or post a note something like "do you want to add this to the companion case" and let me answer "yes" or "no."

That way we are making sure that items needing to be saved to both cases are saved at time of creation all in the same step rather than relying on me remembering to go back and copy to the other case afterward.