First, as I initially assumed, wards that Quirrel cast at Mary’s are rendering them undetectable to patronus communication. That way, if there were no second Harry in Azkaban and third Harry on the way back, McGonagall’s patronus wouldn’t be able to contact him at all.
Second, for all we know about patronus’ methods of travel, it might get autonomously dispatched directly to target’s location, which is normally unique. There are no conservation laws that prohibit patronus splitting into two independent messengers, as there are no conservation laws preventing you from having two mirror reflections, or two acoustic echoes at the same time; and reflections and echoes can be interacted with in magical ways in Potterverse. That means that all two or three copies of Harry can get the same message, give non-conflicting answers, and McGonagall won’t suspect anything.
I want to propose two more possible solutions.
First, as I initially assumed, wards that Quirrel cast at Mary’s are rendering them undetectable to patronus communication. That way, if there were no second Harry in Azkaban and third Harry on the way back, McGonagall’s patronus wouldn’t be able to contact him at all.
Second, for all we know about patronus’ methods of travel, it might get autonomously dispatched directly to target’s location, which is normally unique. There are no conservation laws that prohibit patronus splitting into two independent messengers, as there are no conservation laws preventing you from having two mirror reflections, or two acoustic echoes at the same time; and reflections and echoes can be interacted with in magical ways in Potterverse. That means that all two or three copies of Harry can get the same message, give non-conflicting answers, and McGonagall won’t suspect anything.