If you want 2 instances of AE running at the same time and working on the same comp you need to be rendering to an image sequence not a movie.
If there's audio then you also need to render the audio separately and then put the image sequence and the audio in the media encoder to finish the project as a movie.
I hope you are not editing a long video in a single AE comp. More than 90% of my comps are a single shot under 10 seconds long. I don't know if I have ever created a clump longer than a minute or two in AE except for an occasional color grade on an already edited and rendered to a suitable DI (Digital intermediate) movie.
Thanks but what would happen if i rendered to other than an image sequence? It would break? This is not for making changes just render. Could it work with just a copy of the save file so its two different projects, or is it linked with access / caching of the asset files.