well...first you want to put each picture on its own layer. Then "layer mask" the first picture. Select a gradient going from white to black. Start the gradient on the left end of the picture and drag the gradient all the way to the right end. What should happen is the picture will gradually be revealed from left to right.
Do the same thing with your other picture, except reveal the picture from right to left. The result of the two layers will be a blending effect, which is what you want.