It can be done by app script, by "on form submit" trigger.
Once form A is submitted, it will check if there's a matching record in Form B. If it is found (matching by unique ID), it will compare all the values. If all values are matching, it will do nothing. If not matching, it will send a mail.
Once form B is submitted, it will check if there's a matching record in Form A. If it is found (matching by unique ID), it will compare all the values. If all values are matching, it will do nothing. If not matching, it will send a mail.
I can do it in one day.
I have done much more complicated things in Google Forms.