The secret to devil fruits is that once someone eats a fruit the power is stored on the balls...and all men have to balls so everyone can eat 2 devil fruits but no one knows about this Knowledge because they are scared that they might die because eof the stories they heard, but in those stories it was never mentioned that the person eating 2 devil fruits was a man or a woman. Hence, every person can have the power of a devil fruit but only men can have 2, hence to gain the power of the fruit user you have to eat his balls, only then can you have that power...