double gauss(double *x, double *par) {
  //       0                                  1          2
  //       C                         x        mean       sig
  return par[0]   *   TMath::Gaus(  x[0]  ,  par[1]  ,  par[2]) ;
}

double fitFunc(double *x, double *par) {
  //         0    1    2    3    4    5  
  //  par:  C1    m1   s1   C2   m2   s2
  return   gauss(x,par    ) + gauss(x,&par[3]);
  //return gauss(x,&par[0]) + gauss(x,&par[3]);
}

void double_simple_gauss_fit() {
  delete gRandom;
  gRandom = new TRandom(123456); // Setting the seed

  TH1F *gauss = new TH1F ("gauss", "Double Gauss Distribution", 200, 0, 20);

  double a;
  for(int i=0; i<100000; ++i){
    a=gRandom->Gaus(7,1);
    gauss->Fill(a) ;
    a=gRandom->Gaus(12,1.5);
    gauss->Fill(a);
  }

  TF1 *f1 = new TF1("f1name", fitFunc, 0, 20,6);
  f1->SetLineColor(2);

  // Orismos oriwn kai onomatwn parametrwn prwtis gaussianis
  f1->SetParameter(0,4000); f1->SetParName(0, "Constant 1");
  f1->SetParameter(1,7);    f1->SetParName(1, "Mean_1");
  f1->SetParameter(2,1);    f1->SetParName(2, "Sigma_1");

  // Ovismos oriwn kai onomatwn parametrwn deyteris gaussianis
  f1->SetParameter(3,2500); f1->SetParName(3, "Constant 2");
  f1->SetParameter(4,12) ;  f1->SetParName(4, "Mean_2") ;
  f1->SetParameter(5,1.5);  f1->SetParName(5, "Sigma_2") ;

  gStyle->SetOptFit(111);
  TCanvas *c1 = new TCanvas("c1", "Gauss", 800, 500);
  c1->cd() ;

  gauss->Fit("f1name","","",4,16);
}