A. Two waves propagate in the same medium, and the wave velocities are equal, reaching the midpoint P. Therefore, A is wrong.

B, because the wavelength of A is greater than that of B, the peaks of two waves cannot reach point P at the same time when point P is superimposed, so the maximum displacement of point P cannot reach A 1+A2. So, B is wrong.

When the peaks of C and A reach S2, the peak of B has crossed S 1, so C is wrong.

D. When two waves meet, the two peaks reach the left side of point P at the same time, and the displacement reaches the maximum after superposition. Therefore, when the two waves meet, only one point on the rope can reach A 1+A2, which is on the left side of point P, so D is correct.

So choose D.