"I am listening to the music and he is reading magazines."

Think of the English "while", it can connect two contrasting sentences. Not in the sense of "during" but rather in the sense of "and" (I am a student while he is a salesman= Я студент, а він продавець)

You cannot use it when the statements are the same, sounds strange (I am not a student while he is a student = Я студент, а він студент ~~> Я студент, і він студент.

i means the correspondence of the two actions ( я їм, і він їсть). Reading and listening are not alike, so we connect the two with а (meaning I am listening to music and he (he is not listening to music) is reading magazines