Manual photo merge in PSE 18?

Here is what I do (also see my post on creating moving Gifs which uses a similar method)

Manual method:

• Open two photos
• make one empty canvas large enough to fit both
• paste the two photos into empty canvas as two layers
• use the move function to move the layers while having the blend function in "difference" mode. This will help you to align
• You may have to use free transform to match edges and rotate as neded
• erase the parts you dont want to show
• Trim as needed
• use content aware fill to tidy up the edges

Auto assisted method

• Open two photos
• Use guided merge panorama
• When done use the expert mode, you will see the the two photos as two layers, either erase the mask or manually modify the resulting layer.

MichelParis also suggested using the complementary software called elements+, which specifically adds a lot of merge/stacking functions.