Do they have any common members?
I suppose you could write a class human_being that would have a virtual void give_information(), and that would be the parent class of customer and staff.
Customer(char name[], char gender[], int phone, char pcode[], float dob);
So if I made a human_being class then made staff and customer inherit from there. Customer could have like card number, amount of loyalty points and erm out of ideas now lol