redirect sys.stdout to specific Jupyter Notebook cell

The documentation for ipywidgets.Output has a section about interacting with output widgets from background threads. Using the Output.append_stdout method there is no need for locking. The final cell in this answer can then be replaced with:

def t1_main():
    for i in range(10):
        output1.append_stdout(f'thread1 {i}\n')
        time.sleep(0.5)


def t2_main():
    for i in range(10):
        output2.append_stdout(f'thread2 {i}\n')
        time.sleep(0.5)

output1.clear_output()
output2.clear_output()
        
t1 = Thread(target=t1_main)
t2 = Thread(target=t2_main)
t1.start()
t2.start()
t1.join()
t2.join()