Please either combine these two, or allow separate instances.
Kind of like how you can do PS4/Windows/Ps3
Just allow a separate Steam category, so that people wanting to switch between windows and steam can do so.
As a follow up, the only available help says you just need the *base game* on your chosen platform to get the expansion. You cannot even register the base game to steam.
You also can't switch, even if you have all brand new codes.