One of the big new features in Android 7.0 Nougat is indeed the multi-window mode. It's a feature that users have been begging Google to implement into Android for years and years. Many third-party skins like Touchwiz and Optimus UI have had the feature for quite a few years. Now those that like to stick to stock Android can experience multi-window as well, with Android Nougat. With Nougat now being available, and a bit more stable, app developers are using the API's to create some pretty interesting apps. One of those is Parallel Windows, which does exactly what you think it does.
With multi-window in Android Nougat, you are able to run two apps side-by-side. But they can't be the same app. With Parallel Windows, you can run two instances of the same app side-by-side. Now many may think that this isn't necessary, but think about using Chrome. If you want to have one tab open on one side and another on the other, it's not possible, because it's one app. But with Parallel Windows it is now possible. This app does not require root privileges, so just about anyone can use the app on their Nexus device or another device that runs on Nougat.
As you might expect, with an app that's built off of a brand new feature in a version of Android that just released a few weeks ago, this app is a bit buggy. So if you are looking to use this app and run two windows side-by-side of the same app, be sure to keep that in mind. The developer is definitely working hard on Parallel Windows, so if you do experience any bugs or issues, be sure to report them so they can be fixed. Seeing as if the developer doesn't know that a particular bug exists, then he isn't able to fix it and make the experience better for everyone.
Parallel Windows is a free app on the Google Play Store, there are no in-app purchases either. So you can simply download it and install it and use it as much as you want.