So, when getting separate downloads like that, it downloads like a separate game. So just starting up episode1-2 would be a fresh start. What you can do is find the folder that has your saved game files in episode 1 and copy them into the folder for episode 1-2 that keeps saved files(this folder will be blank at first) and then they'll appear in the load screen when booting up episode 1-2.
Alternatively, you can copy the whole folder for episode 1 saves into your files, uninstall ep 1 and buy&download ep 1 and ep 2, then cut/paste that folder accordingly in the new downloads(both episodes) folders.
The only thing that would keep this from working is if the next 'episode', chapter, whatever, is, in fact, a whole new game itself(not an update; different site page), i.e., they don't have the same game files/coding.
Hope that helps and makes sense.