So decide if this is actually for you before following these steps
It's not shown in user builds, so we have to do some messing. You'll either need root or a custom recovery to do this, but as not everyone wants root, here's how you do it with TWRP, a USB cable, and a PC with ADB:
1. Boot your device into recovery
2. Mount your system by going to Mounts > Tick System
3. On your PC open a command line, somewhere like your desktop is useful
4. Type the following command:
adb pull /system/build.prop
6. Find the "ro.build.type" line
7. Change the text after = to "userdebug" from "user"
BONUS: If you want a nicer DPI here, change the line "ro.sf.lcd_density" to your preferred DPI
8. Save the file
9. Go back to the command line and enter the following command:
adb push build.prop /system/
cd system chmod 644 build.prop
13. When the device has booted, go to the developer options
14. Scroll down to the bottom of the "Drawing" category
15. Enable multi-window and have some fun!
If you're rooted...
1. Download "Build.prop editor"
2. Find "ro.build.type"
3. Edit it to have the value "userdebug" from "user"
4. Reboot when prompted
5. When the device has booted, go to the developer options
6. Scroll down to the bottom of the "Drawing" category
7. Enable multi-window and have some fun!