In principle, it should not be necessarily blocking your main script, but this depends on many things, like what is your OS, how your secundary script is writing into the file, whether it closes the file when it is done, whether your secundary script is permanent or temporary, whether you are waiting for any specific output in the file, too many things that we don'y know.

I also wonder whether this is the best way of doing things. If you want to read the output of the secundary script in your main script, why do you need a secundary script at all? I am not saying it is a bad idea, I don't know enough, I am just questionning whether it is a good idea. Please tell us more about what you are trying to achieve.