I am trying to fit two functions with two data sets simultaneously and the function is basically the real and imaginary part of the Lorentzian, I have the dataset which you can find in the link here Dataset, it has 3 columns, first is frequency, 2nd is the Real part and the 3rd column is the imaginary part.

I have defined my functions as

```
modelImag = ((a*(f^2 - x^2))/((f^2 - x^2)^2 + (d^2*x^2)));
modelReal = (((a*(d*x))/((f^2 - x^2)^2 + ((d)^2*x^2))));
```

Please not that I can fit the my data indiviudually but the paametess value are different for both case, in practice same set of parmeters should fit both functions.

I have taken my dataset as

```
Data = Import("E:\Shelender\codes\Mathematica\Q\a.csv");
dataReal = Data((All, {1, 3}));
dataImag = Data((All, {1, 2}));
data = Flatten({{1, #((1)), #((2))} & /@
dataReal, {0, #((1)), #((2))} & /@ dataImag}, 1)
nlm = NonlinearModelFit(
data, {modelReal + modelImag}, {{a, 1.0*10^7}, {f, 10.0*10^6}, {d,
33000}}, {a, x});
```

I think I have some problem while non-linear model fit.