Technically you can do this (if you have enough memory, which is not certain on Nano), just load the multiple models like you see in detectnet.py / posenet.py and combine those parts into one script. You will want to do your own command-line parsing for the model loading or avoid that, since the command-line parsing in jetson-inference is setup for just one model.
Also, you may be able to use just posenet for your uses, because it can provide you bounding box of the whole human, and you can also look for the face parts of the pose. That would be a lot faster than running three separate models to accomplish essentially the same thing.