Complex sentence: Rhea Silvia conceives the twins by the god Mars, or by the demi-god Hercules; once the twins are born, Amulius has them abandoned to die in the river Tiber.
Split into simpler sentences: Rhea Silvia conceives the twins by the god Mars, or by the demi-god Hercules. Once the twins are born, Amulius has them abandoned to die in the river Tiber.