I would like to share this app, MULTIWINDOW Apps Manager, to easily enable or add more apps to the MULTIWINDOW TRAY.
Please READ the NOTES below for Important Info before installing this app so you will know what it does and it's limitations.
NOTE I: This app requires a ROOTED device. If you don't know what rooted is, then more likely your device is not rooted so please don't install if you are not sure if you have a rooted device.
NOTE II: This app only adds apps you select into the multiwindow app list and has nothing to do with how multiwindow function works. It seems that in EDIT mode, that is when you are adding apps from the edit list to the traybar, the wrong app is being added. This is because Samsung did not make the multiwindow edit list to support more than 32 apps at the same time. This is a shortcoming on the part of the samsung multiwindow code and has nothing to do with this app. So when you enable more than 32 apps and when in EDIT mode, please drag apps you want to include in the Multiwindow Tray from the START of the EDIT app list (in EDIT mode) and NOT from the BOTTOM so you will not have trouble picking the wrong app.
NOTE III: Just because you can add more apps in the Multiwindow traybar does not mean you have to add all your apps. Based on experience, not all apps are multiwindow-friendly like SHEALTH and some others, and adding too many apps causes the multiwindow traybar to lag or slow down so BE WARNED.
NOTE IV: If you have previously edited others.xml or features.xml, you may have introduced some errors to the xml file layout which would prevent this app from properly functioning. Editing the said xml files manually is not advisable due to human error of breaking the xml structure by accident. If this app does not work after previously manually editing the said xml files, then I suggest you restore the original unedited xml files.
DOWNLOAD from PLAY STORE or at the bottom of this post.
1. Make sure your device is ROOTED.
2. Make a Nandroid Backup or all backups that you can think of.
3. Install the app, Run and grant root/superuser/supersu access.
4. Select your apps and reboot.
THANKS to @soloilmeglio and @and2 for testing.