zomb0i Nothing. As long as you have both parts in the same folder, when you extract Part 1 the unpacker automatically fetches Part 2 and unpacks it as well.
You can look up "
multi-volume archives" if you are curious what they are, but for the average Joe the process is simple: put all parts in the same folder, extract Part 1, done. What that "average Joe" tends to F up is they download first part, and try to unpack it while downloading the other parts - that, of course, fails because the unpacker checks for all the other parts, cannot find them, and stops. MVAs are not meant to be unpacked individually.
Actually, in this case the links are interchangeable: for example, you can download part 1 from GoFile and part 2 from PixelDrain, and the game will unpack just fine. The oopsie you ran into, by the way - and even though I am 99.9% sure, this is still just an assumption - was that you probably downloaded Part 2, and later downloaded Part 2
again to the same folder, Windows went "oh, there is a file by that name already" and automatically renamed the new one by adding "(1)" at the end - so you ended up with "7z.001" and "7z(1).002", which meant that the filenames were no longer identical.