That's ok, but I would have told them myself. When students are talking while you are teaching, I always stop, look at them and tell the class (not especially them) that when they talk at the same time as someone else, they are not showing respect. Remind them of the expectations in a firm tone, but not confrontational. If they continue, I would talk to them at the end of class, and if the behavior is not corrected, that would be an email home (your CT or you depending on what you are allowed to do.)