  We theoretically investigate the application of the fringe-locking method
(FLM) in the dual-species quantum test of the weak equivalence principle (WEP).
With the FLM, the measurement is performed invariably at the midfringe, and the
extraction of the phase shift for atom interferometers is linearized. For the
simultaneous interferometers, this linearization enables a good common-mode
rejection of vibration noise, which is usually the main limit for high
precision WEP tests of dual-species kind. We note that this method also allows
for an unbiased determination of the gravity accelerations difference, which
meanwhile is readily to be implemented.
