Open the two photos you wish to make a mask with in separate windows.
Select > All , one of the images.
Then Edit > Copy.
Then Edit > Paste
The two images should now be on separate layers in the same document. Check your layers palette to make sure.
If one the pasted image needs to be resized press Control T to transform. When happy with changes hit enter.
In the Layers Palette, make sure the top layer is selected.
Click Add Layer Mask at bottom of Layer Palette.
Layer Mask thumbnail appears next to Layer 1 thumbnail.
Select the Gradient Tool
Double click Edit Gradient icon at top of screen.
Make sure that the gradient is black to white.
Select Linear Gradient Icon in Control Panel
Click and drag on Desktop.
See what happens!
Try shorter drags, longer drags
Try the Radial Gradient