Phase Coincidence:

Phase coincidence means that the two waves start at precisely the similar moment. They are "lined up." This is shown in figure below for two waves having dissimilar amplitudes. (When the amplitudes were similar, you would view only one wave.) The phase difference in this situation is 0°.

When two sine waves are in phase coincidence, the peak amplitude of the resultant wave, that also will be a sine wave, is equivalent to the sum of the peak amplitudes of the two composite waves. The phase of the resulting is similar as that of the composite waves.

Figure: Two sine waves in phase coincidence.