Multi-window has always been one of the highlighted features of OmniROM. The feature is extremely useful for a limited use case scenario and has been the primary reason of many people flashing custom ROMs on their devices.
Now, thanks to the power of Xposed framework and the hard work of developer LovewuChin, even stock Android users can now enjoy multi-window without flashing any custom ROM using the XMultiWindow module. The module is still in early stages of development and is a bit buggy but it definitely has a lot of promise. The developer has also been working relentlessly on fixing the bugs and improving the overall usability of the module.
Currently, the Xposed module only works on AOSP, MIUI and AOSP based ROMs. Chances of the module working on OEM based skins are pretty bleak, because of closed source framework and libraries used by OEMs.
If you are not already using Xposed framework, head over to this guide to know what is Xposed and how to install it. You can download the XMultiWindow module from the Xposed installer app on your Android device or directly from the Xposed repository.