Don't upgrade to Windows 10 just yet if you don't want to. You don't need to be running Microsoft's latest operating system in order to benefit from multiple graphics card support with Vulkan. It turns out that the Khronos group which is developing the graphics API, is making sure that its multiple GPU support works on older operating systems like Windows 7 and 8.1 too.
"The Vulkan multi-GPU specification is very definitely NOT tied to Windows 10," it said in a statement. "It is possible to implement the Vulkan multi-GPU extension on any desktop OS, including Windows 7, 8.x, and 10 and Linux."
The reason for the confusion it turns out, is because previously Khronos mentioned that Windows Display Driver Model would be needed for multiple graphics cards to work correctly together. That would mean that there would be no Linux support for multi-GPUs with Vulkan, which isn't true either. It only uses WDDM when running Windows.
On top of that, it confirmed that while LDA mode would be used in some instances, it wouldn't be a necessity, but was supported by Windows 7 and 8.1 any how, so shouldn't prove a problem.
This is why developers like Cloud Imperium have dropped DirectX12 for Vulkan, because it seems like the most forward thinking API at this time.
In a statement, Cloud Imperium developer Ali brown said (via PCgamer): "Years ago we stated our intention to support DX12, but since the introduction of Vulkan which has the same feature set and performance advantages this seemed a much more logical rendering API to use as it doesn't force our users to upgrade to Windows 10 and opens the door for a single graphics API that could be used on all Windows 7, 8, 10, and Linux."