Can only happen with more than 2 data select with their associated model. Can only operate with the park engine selected Fitting on the menu. check park.
Highlight the data to fit. Select from the context menu Select Data for fitting. A new tab with the name of the data selected will appears. Select a model used for the fit from the combo box. Model's parameters will be display.Set initial parameters if need. Check at least one parameter to fit.Go to Simultaneous Fit and check at least 2 models name to fit.Click on constraint fit button. results will be displayed on tabs corresponding to each model select.
Perform Simultaneous Fit without constraint step. Enter constraint in the text control next to constraint fit button. Constraint should be of type model1 parameter name = f(model2 parameter name)for example, M0.radius=2*M1.radius. Many constraints can be entered for a single fit.Each of them should be separated by a newline charater or ";"