Not sure I know what a mag is. But if their both mobile inverts I doubt they would settle next to each other. And if they did most likely one would move after being stung
I have both in my tank fir years and Never had an issue with the 2 fight. Not saying it can't happen. But I wouldn't worry about your bta and mag fighting to the death
I saw a chart about 18 months ago that showed which nems were more likely to be or not be successful when housed together, but for the life of me cannot find it now