To observe stationary interference pattern formed by two light waves, it is not necessary that they must have

For a stationary interference, there is maxima and minima at a stationary positions. For that , phase difference should be constant, and the sources must be coherent ,so they  should have same frequency.

When Amplitude is same, the minima that is formed is completely dark but it id not necessary condition.

We know $I\propto {\left(Amplitude\right)}^2$