I investigated it a bit more. I don’t know if the trick works on linux, but there’s info on how to do it on windows in this other thread: Eevee and multiple graphics cards
To use this with b-renderon you must have a couple of different copies of blender in your pc, and via the nvidia application assign different gpus to the different blender executables. Then in B-renderon you set your different files to be rendered with the different copies of blender. Currently b-renderon let’s you choose the blender path and it looks for the “blender” binary there, but in a coming release I’ll make it so that it let’s you choose binaries, so you can use different blender executables in a same blender folder.
As for the temporary workaround that you mention, it would work for cycles choosing different devices, which is a property of the scene. Changing the preference would also work as long as the new preference isn’t saved at the moment b-renderon calls blender.
Hope my reply is not too confusing, english is not my first language!