Multi-GPU for gaming has always been a waste honestly: expensive, supported by few games, and often unstable. For ED I believe it is totally broken in Odyssey/4.0, and there's zero chance of it being fixed because it's a lot of effort for a single digit amount of users. Neither AMD or NVIDIA support CrossFire/SLI anymore either, recommending Vulkan/D3D12 multi-GPU, which is even harder to develop for.
So upgrade your GPU instead of buying another.