  We have studied microwave photoresistance in a two-dimensional electron
system subject to two radiation fields (frequencies $\omega_1$ and $\omega_2$)
using quantum kinetic equation. We have found that when $\omega_2/\omega_1= 1 +
2/N$, where $N$ is an integer, and both waves have the same polarization, the
displacement mechanism gives rise to a new, phase-sensitive photoresistance.
This photoresistance oscillates with the magnetic field and can be a good
fraction of the total photoresistance under typical experimental conditions.
The inelastic mechanism, on the other hand, gives zero phase-sensitive
photoresistance if the radiation fields are circularly polarized.
