Hello there,
Here is my personal experience. Take it as a grain of salt if you wish, but it might save you days or months of learning.
Its possible to flash all adapters, but it usually does not work. Best way is to build a script in vbscript, batch file, or powershell. Flash each adapter individually.;
Run it as administrator.
I personally just run a batch file as administrator for each adapter.
This procedure can unlock the rom and flash it.
One I flash each rom, I then uninstall any 3rd party GPU utility such as MSI afterburner (make sure to select delete all settings), then uninstall amd drivers.
Then run the amd driver wipe utility as some AMD driver version + 3rd party tools like MSI AB will mess with voltage, fan rpm, mhz, etc.
As a contingency, you may to have remote desktop or some sort of remote connectivity program installed. This would be in case you flash bad roms on all of your adapters.
Then do a clean install of an amd driver package. Ideally you want something that allows use of unsigned drivers (modified bios roms). Personally I use 16.9.2 (i think that's correct version).
Best of luck and have fun!
EDIT: Eliovp's method is great Or you can just have a copy of the script for each card, or a script that flashes each card one by one.
I prefer one by one as I have flashed bad bios before when testing.