This is one of the best compliments ever, thank you! Double exposure photography has long been one of my favorites.
To answer your question: It’s Photoshop. If your going to be making a double of a landscape and a portrait, the trick is to find your landscape first. Then model your portrait so that it compliments the landscape.
As far as editing goes, I do a lot of blending with layers. I suggest starting by opening the landscape image, placing a portrait over top, and then making the portrait layers blend mode “soft light.” From there you can use a large soft paint brush on the portrait to blend them together.
Hope this helps and thanks for stopping by! I love the questions.