This is probably also pretty easy with some CLI tool.

This is one of the few image tasks I do on the CLI xD

Stack two images horizontally (left and right)

convert a.jpg b.jpg +append horizontal.jpg

Stack two images vertically (top and bottom)

convert a.jpg b.jpg -append vertical.jpg

Images are not the same dimensions? Use gravity to align them at the center and make the unused space transparent

convert a.jpg b.jpg -background transparent -gravity center +append horizontal.png