I was responding on my phone before, so I left the following for now.
I don't think it's spelled out explicitly, so take it as you like it. On the other hand, it is explicitly stated that the individual Gift is different for each person. People don't become used to the Gift, they become used to the people who have the Gift. People who are around the Gift for a long time learn to recognize it for what it is (think Redcaps, and the covenfolk that have direct interaction with multiple magi on a frequent basis). Stick four young people in a room, with some nitwit in front of them chattering away, they're going to be spending most of their time thinking about the people around them, and the nitwit up front is concerned for his own safety because of the homocidal maniac, the sex fiend, the deviant and the creepy quiet one in the corner who never stops staring at him. If you don't want it to stack, then you don't want it to stack. And in that case, why aren't other covenants doing it? Your stated objective was to be simulationist and "to identify any gaping holes that would blow verisimilitude out of the water." Why is your covenant the first to do this? One thing that supports the idea that penalties from multiple Gifted children stack, is how Gifted children in the Theban Tribunal aren't all placed in one central location, and are instead sent to various magi throughout the Tribunal until the next septennial gathering where the Gifted children select masters. If it wasn't a problem, I'm sure that there would be a boarding school and they would all be sent to that one location. Would make life much easier for everyone, I'm sure.