Assuming you've already installed Xposed and know how it works.
Now download the Halo for SGS app and install it.
- Download and install the module.
- Activate the module.
- Perform a soft reboot.
- Open this app.
- Select the apps to be shown in multi window mode.
- And just run it.
If you ask me I don't find this much productive.
As you already know how small Star's screen is
, it can barely view a single app.
I don't think it'll be a good idea to use multi-window on a 3 inch screen.
But hey, it doesn't hurt to try now, does it ?