> joining them together in software

What if they were joined together by exposing 2 different overlapping film negatives?

You may enjoy "Faking it: Manipulated Photography Before Photoshop"

https://www.google.com/books/edition/Faking_it/nGvTg_HC32YC?...

> get the mountains you want and the rocks and people you want

Digital painting? Too difficult, maybe just use Firefly "Adobe's online AI image generator" ;-)