Charles.Surge: Do as you like, but I think that doesn't really make sense. The "bundle" is discounted on Steam, but it's the same price as here on GOG and when you buy DLC on GOG, that content is just added to the single library entry for the main game. So, as long as GOG doesn't screw anything up the only difference in the end from buying the two items seperately versus the one item "bundle" is that it shows as two items on your order records and maybe how your "ownership" is reflected on the different game pages.
Since I'm posting: on the Steam announcement of this sale the devs also say they're working on another expansion.
See? :) Btw I already got burned by accidentally buying a game and expansion upgrade separately for example, but the complete version doesn't shows up as owned. And I also bought games thinking they are done releasing DLCs. Now I just wait until I'm convinced that nothing more is coming.