from twisted.internet import defer, reactor
import time


@defer.inlineCallbacks
def async_task(*args, **kwargs):
    print('async_task started')
    yield some_fast_async_job()
    reactor.callLater(0, another_async_task, *args, **kwargs)
    print('another_async_task has launched')
    print('async_task finished')

@defer.inlineCallbacks
def another_async_task(*args, **kwargs):
    print('another_async_task started')
    yield some_slow_async_job()
    yield some_another_async_job()
    print('another_async_task finished')

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OUTPUT:

async_task started
another_async_task has launched
async_task finished
another_async_task started
another_async_task finished