I’m not sure, what your work flow would be and how you would like to force the submodels to predict only a part of the face.
Would you want to use different label sets, i.e. one model would only get nose - not nose labels, while the others would get the mouth pairs etc.?
Do you have a dataset, where only parts of faces would be visible?
If the complete face is always visible, you wouldn’t be able to only force on a particular part.
Or would you only use cropped patches of the face image?
In that case, I’m not sure, how your validation/test set would look like.