Call EXE2 only when EXE1 has completed the task

I have a python script file (which will be later converted to an exe , lets say EXE3)

My codes in script file for EXE3 reads as such:

import os

os.startfile('\file_path\EXE1')
os.startfile('\file_path\EXE2')

I expect that EXE2 should execute only when EXE1 has completed its task and the output from EXE1 is now ready to be used in EXE2.

However, what I see is that EXE1 and EXE2 occur simulataneoulsy.

Can somebody plz let me know how do I control the time of execution of EXE2 which is dependent on EXE1 ?