A two-Lorentzian model.

   This is simply the sum of two lorentzian functions in some part of the
   spectrum. Each individual Lorentzian has its own peak frequency, area, hwhm
   and phase, but they share common offset and drift parameters.

def two_lorentzian(freq, freq0_1, freq0_2, area1, area2, hwhm1, hwhm2, phase1,
                   phase2, offset, drift):
   """
   A two-Lorentzian model.

   This is simply the sum of two lorentzian functions in some part of the
   spectrum. Each individual Lorentzian has its own peak frequency, area, hwhm
   and phase, but they share common offset and drift parameters.
   
   """
   return (lorentzian(freq, freq0_1, area1, hwhm1, phase1, offset, drift) +
           lorentzian(freq, freq0_2, area2, hwhm2, phase2, offset, drift))