Update: In a recent Reddit Ask Me Anything, Andrew from the Pixel C team at Google confirmed that native multi-window support is coming in Android N, which will be previewed at Google I/O 2016 and released later in the year when the two new Nexus devices come out.
All you need to do is change a single field in "ro.build.type".
Prerequisites
> A rooted phone running Android 6.0 Marshmallow
> A root file manager with text editor (like ES File Explorer) or Build Prop Editor BuildProp Editor
How to enable multi-window mode on Android 6.0 Marshmallow
1. Open Build Prop Editor and scroll down to ro.build.type. Tap it and change the Property Value from user to userdebug.
2. Tap Save and then Yes to reboot your phone (this is necessary to apply the changes).
Change the Property Value to "userdebug", save your changes and reboot. 
Once you've edited the build.prop file, multi-window mode appears in Developer Options. Using multi-window mode on Android 6.0 Marshmallow
In your multi-tasking view (or recent apps list), you'll now see a square bracket in the upper right of each app card. Tap the target to choose the split-screen view you want: upper half, lower half or full screen. Note that top and bottom switch to left and right in landscape mode.
In landscape mode, things go left/right, but occasionally you'll see some glitches. Multi-window mode on Marshmallow works surprisingly well for a feature that was left out. It's a bit clunky and occasionally you get weird overlaps, like as shown the upper left of the image above, but generally speaking, it works fine.
Unfortunately, you can't re-size the split-screen windows, but that's what you get from an unfinished developer feature. It's not perfect, but it is there and it is working – and that's half the battle. [post_ad]
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