Are these multiple networks connected somehow to each other, i.e. model1 feeds its output to model2?
If so, could you post an explanation of your workflow?
If the networks are completely standalone models, you could run multiple scripts, specifying the GPU which should be used with: CUDA_VISIBLE_DEVICES=device_id python script.py, where device_id has to be set to the appropriate GPU id.
You could also set the device in your script with:
import os
os.environ['CUDA_VISIBLE_DEVICES'] = '0'