For the sustained interference of light, the necessary condition is that the two sources should:

In case of observable interference, the intensity needs to be remain same $(I = I_1+I_2+2\sqrt{I_1{I}_2} \mathrm{cos\varphi })$, and this is possible when the phase difference $\varphi$ at a given point does not vary with time, so phase difference between two sources must be constant. Coherent sources produce sustained or observable interference as coherent sources have zero or constant phase difference.