  We present a new scheme for measuring the differential phase in dual atom
interferometers. The magnetic field is modulated in one interferometer, and the
differential phase can be extracted without measuring the amplitude of the
magnetic field by combining the ellipse and linear fitting methods. The gravity
gradient measurements are discussed based on dual atom interferometers.
Numerical simulation shows that the systematic error of the differential phase
measurement is largely decreased when the duration of the magnetic field is
symmetrically modulated. This combined fitting scheme has a high accuracy for
measuring an arbitrary differential phase in dual atom interferometers.
