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In some cases you may want to fit the data with two Gauss or Lorentz peaks with the same width but different positions and amplitudes, for example. You can do this in two ways: by specifying custom Fit Curve with your equation or by joining the 'width' parameters of two peaks.
To join parameters of two or more Fit Curves select one of desired Fit Curves, then select desired parameter in parameters table and press Join button in the bottom of the panel (or double click on parameter).
You can specify the selected parameters as equal or proportional by entering multiplier and constant for each parameter.
Joined parameters are shown with blue color (instead of black) in curve parameters table.