POC

Build the base images

docker build -f base_image.Dockerfile -t python-base-image .  
docker build -f other_base_image.Dockerfile -t other-base-image .

Build the inheriting image

docker build -f inheriting_image.Dockerfile -t inheriting-image . 

Run a container of the inheriting image

docker run -it inheriting-image

Note, that packages from both requirement files are available.

Caveats

Pip exists for a reason. Dependency conflicts wont be checked when simply copying packages. After copying the images it would be reasonable to check for conflicts using pip check