This addon is pretty much still in Alpha and I still need to test it for every edge case. So far I have tested it with older Nvidia (GTX 10 series) with an Intel igpu and a dual Intel B580 setup. Note that for laptops using Intel + older Nvidia GPUs there is an extra option to enable, since those require a different flag to change the GPU (currently I do not think dual GTX 10 series will work with this addon, you can thank Nvidia for purposefully being in the way of open source drivers for that one). For Nvidia desktop using RTX 20 or newer with open source drivers would be recommended.
In theory this addon should work with any OpenGL/Vulkan rendering engine (most testing was done with Blender 4.2+ and Eevee). But thanks to the testing of one of my friends, I found that some render engine plugins require an active GUI in order to work. I have a work around in mind already which will be implemented at a later date. Cycles or other compute based rendering engines like Indigo or Lux are not supported, you should use their internal logic for multi gpu rendering.
Single image rendering is not implemented but is under consideration (I roughly think I know how but by taking a tiling like approach for each gpu but need some time to implement and test it).
Currently I only tested rendering video files with mp4 and mkv. Image formats should work without issue. Ffmpeg is a psuedo requirement when rendering video files, since it is used to concatenate the video file of each gpu together but not having it won't stop the video from being rendered. If you don't have it installed the videos can be found in /tmp/ as output_mgpu_*.mp3, the number corresponds to the correct order they should be imported.
TLDR: use this plugin for multi gpu rendering only with OpenGL/Vulkan (preferably Eevee) engines and only for animations (for now), Nvidia users preferably have RTX 20 or newer and use Open source drivers. Install ffmpeg.
Windows lacks an equivalent to DRI-PRIME which this addon leverages to start multiple render instances, while it is possible to tie a GPU to an instance of a program, this has to be done through the GUI. If Windows one day adds an equivalent like that for scripting I can update the addon for it, but for now Windows is unsupported.