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.