Operator Overload, pick and reassign two values created in the class object

I don´t know how to reassign the value of two -in main- created class objects in order to reuse them inside an operator overload.

My operation overload should sum two numbers treating the nominator and denominator separately.

//This is my Declaration:
public:
Fraction operator+ (Fraction& obj);
//...
private:
int nenner;
int zaehler;

//This is what I tried in the Implementation:

Fraction::operator+(Fraction& obj, Fraction& obj2) 
{
   Fraction res;
   int nenner1 = obj.getNenner();
   int nenner2 = obj2.getNenner();
   int zaehler1 = obj.getZaehler();
   int zaehler2 = obj2.getZaehler();   
   int newNenner = Fraction::kgV(nenner1, nenner2);
   int newZaehler = zaehler1 * (newNenner / nenner1)+ zaehler1*(newNenner / nenner2);
   res = Fraction(newZaehler, newNenner);
   return res;
 }