I've been looking into a fix to allow the usage of multiple dual shock 4 controllers for multiplayer gaming and came up with nothing that worked on the Amazon Fire TV until now.
Since we aren't able to change the IME setup, there's an app that lets you keymap any device connected either with USB or Bluetooth. Its called Tincore KeyMapper and while it may come off a bit complicated at first and it has its visual bugs running on the amazon box, it works perfectly.
first install the app and run it. You must be rooted for it to work!
I also recommend you have a mouse to navigate easier and have all the controllers you wish to configure connected and/or plugged in from the start. Any new controllers added afterwards will swap "ports" with the other controllers and will occasionally switch players once a device rescan is done.
once its started create a new profile call it whatever you like ie. openBOR settings
click the magnifying glass(search) make sure to have System Internal and USB/BT *ROOT* selected then hit accept.
You should see all your devices that are connected to the box.
look for your controller you have connected and change the dpad, left stick and buttons to "click keys". You wont be able to make changes until you restart, so exit the app by clicking the 3 dot menu and scrolling all the way down to the bottom and clicking the the last item that isn't visible.
Re run the app and now you can set the keys for the found devices. Enjoy!
There's other cool features if you buy this app, and I plan on buying just because the app works even without IME setups. And the more support we give him, maybe he will make a native port for us in the future.