Dear experts,

I am currently trying to do a simultaneous fit in several variables/distributions in order to extract the number of events of given processes.

The first example [1] I came up with uses two distributions (in this example they are identical):

histograms_1 & histograms_2.

Further for each distribution I have the observed data, a signal template/pdf and a background template/pdf.

I use the latter to construct two models. So far everything works.

Now I would like to combine the two observed data samples:

```
combined_data = deepcopy( roofit_histograms_1['data'] )
combined_data.add( roofit_histograms_2['data'] )
```

where roofit_histograms_2[‘data’] is a RooDataHist.

Next I try to fit to this combined data:

which fails with

```
[#0] ERROR:InputArguments -- RooTreeData::split(data) ERROR category sample doesn't depend on any variable in this dataset
[#0] ERROR:Fitting -- RooAbsTestStatistic::initSimMode(nll_simPdf_data) ERROR: index category of simultaneous pdf is missing in dataset, aborting
```

Obviously something is wrong with the way I create the combined data.

What is the correct way? How do I combine the two RooDataHist such that RooSimultaneous::fitTo is happy with it?

Thank you in advance.

Luke