Put them together by choose 1 image layer then shift+click and drag to other image
Step3
Select one image and add layer mask. To create the layer mask, make sure the select layer (Layer 1) is selected in the Layers palette, then select Layer > Add Layer Mask > Reveal All
Step4
Fill the Layer Mask with a Gradient. Select the Linear gradient type and, from the drop-down colour box, choose the Black, White. click and drag in the document window from the bottom right of the image to the top left, to create the gradient fill.