@shared_task(ignore_result=True)
def update():
    group([app1.tasks.update.s(), app2.tasks.update.s()])()

Received task: main.tasks.update[1a26fb7c-...]
Received task: app2.tasks.update[9c183d56-...]
Task main.tasks.update[1a26fb7c-...] succeeded in 0.072...s: None
Task app2.tasks.update[9c183d56-...] succeeded in 2.000...s: None

proj/app1/tasks
proj/app2/tasks
proj/main/tasks

# main/tasks.py
from celery import shared_task, group
import app1.tasks
import app2.tasks
@shared_task(ignore_result=True)
def update():
    group([app1.tasks.update.s(), app2.tasks.update.s()])()

# app1/tasks.py
@shared_task(ignore_result=True)
def update():
    sleep(5)

# app2/tasks.py
@shared_task(ignore_result=True)
def update():
    sleep(2)

celery -A main worker -l info

Received task: main.tasks.update[1a26fb7c-...]
Received task: app2.tasks.update[9c183d56-...]
Task main.tasks.update[1a26fb7c-...] succeeded in 0.072...s: None
Task app2.tasks.update[9c183d56-...] succeeded in 2.000...s: None

# main/tasks.py
@shared_task(ignore_result=True)
def update():
    group([app2.tasks.update.s(), app1.tasks.update.s()])()

Received task: main.tasks.update[231b7bd1-...]
Received task: app1.tasks.update[3491961a-...]
Task main.tasks.update[231b7bd1-...] succeeded in 0.072...s: None
Task app1.tasks.update[3491961a-...] succeeded in 5.000...s: None

# main/tasks.py
@shared_task(ignore_result=True)
def update():
    app2.tasks.update.s().delay()
    app1.tasks.update.s().delay()