Docker multiple network issues

# test1/docker-compose.yml
version: '2'
networks:
  back:
    driver: bridge
  front:
    driver: bridge
services:
  web:
    image: nginx
    networks:
      - front
      - back
  app:
    image: nginx
    networks:
      - back

# test2/docker-compose.yml
version: '2'
networks:
  front:
    external:
      name: test1_front
services:
  proxy:
    image: nginx
    networks:
      - front