Hello. The two apps can be used side by side, the can both be installed on the same device. The problem was that Android 4.3 was required. I uploaded a new version of the app to the Play Store that should now work on Android 4.0.3 devices and newer. Should be ready for download and install now.
Since you already know the interface of Text Fiction, here is a quick highlight of what to look for in the “for Thunderword” enhancements:
When you pick a story to load, if you pick the text name of the story to the left, “Bronze”, it should run it local inside Text Fiction’s ZPlet engine. In addition, on the right side of each story name there is now a Thunderword icon that I circled in the screen shot. If you touch that icon it will run app to app using the Thunderword engines (Git, Gluxe, Nitfol, Bocfel, Frotz, etc). You also have a third choice, to long-press the Thunderword icon to launch Thunderword’s native rendering. I hope this helps.