  Two-point microrheology measurements from widely separated colloidal
particles approach the bulk viscosity of the host medium more reliably than
corresponding single point measurements. In addition, active microrheology
offers the advantage of enhanced signal to noise over passive techniques.
Recently, we reported the observation of a motional resonance induced in a
probe particle in dual-trap optical tweezers when the control particle was
driven externally [Paul et al. Phys. Rev. E {\bf 96}, 050102(R) (2017)]. We now
demonstrate that the amplitude and phase characteristics of the motional
resonance can be used as a sensitive tool for active two-point microrheology to
measure the viscosity of a viscous fluid. Thus, we measure the viscosity of
viscous liquids from both the amplitude and phase response of the resonance,
and demonstrate that the zero-crossing of the phase response of the probe
particle with respect to the external drive is superior compared to the
amplitude response in measuring viscosity at large particle separations. We
compare our viscosity measurements with that using a commercial rheometer and
obtain an agreement $\sim1\%$. The method can be extended to viscoelastic
material where the frequency dependence of the resonance may provide further
accuracy for active microrheological measurements.
