So I can't get both a Steam and a GOG key? I gotta say, for something that presumably has a marginal cost to you of near-zero (but I spent quite a bit of money on), that's...disappointing.
That would mean you'd get two copies of the game tho...
...however, if what you meant is that you're getting multiple digital copies of the game with your reward tier, I'd think you can choose the platform for each one? I don't have that so can't check, but are they listed as separate entries in your backer portal?
I don't think (at least in this case) that being able to download the game from two different places is really equivalent to two different copies of the game. I'm already getting a physical DRM-free copy of the game if it's a "but you can give a key away to someone else!" issue - and if I choose GOG, I'm already able to get a digital DRM-free copy. I'd just like to be able to have the game on both my GOG and Steam "shelves" (and no, manually adding a non-Steam game to the client is not the same).