Open both photos in Photoshop
Select the Move Tool
Drag one photo into the other document, or copy and paste it into the same canvas
Use Free Transform to resize and position the second photo
Add a layer mask to the top photo
Use a soft brush on the mask to blend the images together
Adjust opacity if needed
Use alignment tools if the photos need to match up
Refine the edges with brush, gradient, or selection tools
Merge visible layers when finished
Save the file in PSD format to keep layers
Export the final image as JPG, PNG, or another desired format