When I have, for example, a requirements-dev.txt and a requirements.txt, I know I can have -r requirements.txt inside requirements-d
requirements-dev.txt
requirements.txt
-r requirements.txt
requirements-d
pip install -r requirements.txt -t /path/to/install
This should work. It worked for me.
If you want different modules to be installed to different locations, then I think you might have to put them into multiple requirements text files. This is at least as far as I know