> 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" ;-)