Use APKPure App
Get YT Multi-Window Fix old version APK for Android
A fix for the YouTube bug on the Android N Preview
There is currently a bug on the Android N Preview that prevents you from watching videos and using another app at the same time (the video stops playing). This is a temporary solution to fix that.
How it works:
Open the YouTube app in a separate window just as you would if you were going to play a video through it. Choose your video and then go to the share menu in the YouTube app and share it to this app. Then your video will start playing and you can interact with another app.
NOTE: This is only intended to be used on the N preview, it does not bring multi-window to other versions of Android