Here is the code we want to show.

pdf->plotOn(plot); 
//draw the two separate pdf's
pdf->plotOn(plot, RooFit::Components("bkg_pdf"), RooFit::LineStyle(kDashed) );
pdf->plotOn(plot, RooFit::Components("sig_pdf"), RooFit::LineColor(kRed), RooFit::LineStyle(kDashed) );
plot->Draw();   // to show the RooPlot in the current ROOT Canvas

using namespace RooFit; 

void CombinedModel() { 

   RooWorkspace w("w"); 

   w.factory("Exponential:bkg1_pdf(x[0,10], a1[-0.5,-2,-0.2])");
   w.factory("Gaussian:sig_pdf(x, mass[2], sigma[0.3])");
   
   w.factory("prod:nsig1(mu[1,0,5],xsec1[30])");

   w.factory("SUM:model1(nsig1*sig_pdf, nbkg1[1000,0,10000]*bkg1_pdf)");  // for extended model

   w.factory("Exponential:bkg2_pdf(x, a2[-1,-2,-0.2])");

   w.factory("prod:nsig2(mu,xsec2[20])");
   w.factory("SUM:model2(nsig2*sig_pdf, nbkg2[100,0,10000]*bkg2_pdf)");  // for extended model

   // Create discrete observable to label channels
   w.factory("index[channel1,channel2]");

   // Create joint pdf (RooSimultaneous)
   w.factory("SIMUL:jointModel(index,channel1=model1,channel2=model2)");

   RooAbsPdf * pdf = w.pdf("jointModel");
   RooRealVar * x = w.var("x");  // the observable
   RooCategory * index = w.cat("index");  // the category


   // generate the data (since it is exetended we can generate the global model
 
   // use fixed random numbers for reproducibility
   RooRandom::randomGenerator()->SetSeed(111);
   // generate binned 
   // plot the generate data in 50 bins (default is 100) 
   x->setBins(50);

   // generate events of joint model 
   // NB need to add also category as observable
   RooDataSet * data = pdf->generate( RooArgSet(*x,*index));  // will generate accordint to total S+B events
   data->SetName("data");
   w.import(*data);

   data->Print(); 


   RooPlot * plot1 = x->frame();
   RooPlot * plot2 = x->frame();
   data->plotOn(plot1,RooFit::Cut("index==index::channel1"));
   data->plotOn(plot2,RooFit::Cut("index==index::channel2"));
    // plot->Draw();

   RooFitResult * r = pdf->fitTo(*data, RooFit::Save(true), RooFit::Minimizer("Minuit2","Migrad"));
   r->Print();

   pdf->plotOn(plot1,RooFit::ProjWData(*data),RooFit::Slice(*w.cat("index"),"channel1"));
   pdf->plotOn(plot2,RooFit::ProjWData(*data),RooFit::Slice(*w.cat("index"),"channel2"));
  //draw the two separate pdf's
   pdf->paramOn(plot1);
   c1 = new TCanvas();
   c1->Divide(1,2);
   c1->cd(1); plot1->Draw(); 
   c1->cd(2); plot2->Draw();



  RooStats::ModelConfig mc("ModelConfig",&w);
   mc.SetPdf(*pdf);
   mc.SetParametersOfInterest(*w.var("mu"));
   mc.SetObservables(RooArgSet(*w.var("x"),*w.cat("index")));

   // define set of nuisance parameters
   w.defineSet("nuisParams","a1,nbkg1,a2,nbkg2");

   mc.SetNuisanceParameters(*w.set("nuisParams"));

   // set also the snapshot
   mc.SetSnapshot(*w.var("mu"));

   // import model in the workspace 
   w.import(mc);

   // write the workspace in the file
   TString fileName = "CombinedModel.root";
   w.writeToFile(fileName,true);
   cout << "model written to file " << fileName << endl;
}

using namespace RooFit; 

void CombinedModel() { 

   RooWorkspace w("w"); 

   w.factory("Exponential:bkg1_pdf(x[0,10], a1[-0.5,-2,-0.2])");
   w.factory("Gaussian:sig_pdf(x, mass[2], sigma[0.3])");
   
   w.factory("prod:nsig1(mu[1,0,5],xsec1[30])");

   w.factory("SUM:model1(nsig1*sig_pdf, nbkg1[1000,0,10000]*bkg1_pdf)");  // for extended model

   w.factory("Exponential:bkg2_pdf(x, a2[-1,-2,-0.2])");

   w.factory("prod:nsig2(mu,xsec2[20])");
   w.factory("SUM:model2(nsig2*sig_pdf, nbkg2[100,0,10000]*bkg2_pdf)");  // for extended model

   // Create discrete observable to label channels
   w.factory("index[channel1,channel2]");

   // Create joint pdf (RooSimultaneous)
   w.factory("SIMUL:jointModel(index,channel1=model1,channel2=model2)");

   RooAbsPdf * pdf = w.pdf("jointModel");
   RooRealVar * x = w.var("x");  // the observable
   RooCategory * index = w.cat("index");  // the category


   // generate the data (since it is exetended we can generate the global model
 
   // use fixed random numbers for reproducibility
   RooRandom::randomGenerator()->SetSeed(111);
   // generate binned 
   // plot the generate data in 50 bins (default is 100) 
   x->setBins(50);

   // generate events of joint model 
   // NB need to add also category as observable
   RooDataSet * data = pdf->generate( RooArgSet(*x,*index));  // will generate accordint to total S+B events
   data->SetName("data");
   w.import(*data);

   data->Print(); 


   RooPlot * plot1 = x->frame();
   RooPlot * plot2 = x->frame();
   data->plotOn(plot1,RooFit::Cut("index==index::channel1"));
   data->plotOn(plot2,RooFit::Cut("index==index::channel2"));
    // plot->Draw();

   RooFitResult * r = pdf->fitTo(*data, RooFit::Save(true), RooFit::Minimizer("Minuit2","Migrad"));
   r->Print();

   pdf->plotOn(plot1,RooFit::ProjWData(*data),RooFit::Slice(*w.cat("index"),"channel1"));
   pdf->plotOn(plot2,RooFit::ProjWData(*data),RooFit::Slice(*w.cat("index"),"channel2"));
  //draw the two separate pdf's
   pdf->paramOn(plot1);
   c1 = new TCanvas();
   c1->Divide(1,2);
   c1->cd(1); plot1->Draw(); 
   c1->cd(2); plot2->Draw();



  RooStats::ModelConfig mc("ModelConfig",&w);
   mc.SetPdf(*pdf);
   mc.SetParametersOfInterest(*w.var("mu"));
   mc.SetObservables(RooArgSet(*w.var("x"),*w.cat("index")));

   // define set of nuisance parameters
   w.defineSet("nuisParams","a1,nbkg1,a2,nbkg2");

   mc.SetNuisanceParameters(*w.set("nuisParams"));

   // set also the snapshot
   mc.SetSnapshot(*w.var("mu"));

   // import model in the workspace 
   w.import(mc);

   // write the workspace in the file
   TString fileName = "CombinedModel.root";
   w.writeToFile(fileName,true);
   cout << "model written to file " << fileName << endl;
}