The major issue you have is that all writes within an AppGameKit app are local and sand boxed, which means the file resides in a protected app-centric location. You cannot go and copy that file to some other part of the device as that would violate the safety of a sand box (i.e. you don't want AppGameKit apps writing all over your device now). The solution is to create a central server application that runs on-line 24/7, and which will receive your text data in the form of a network packet. Your server can store the text file locally at the hub, and then send that file out to another other AppGameKit app that might request it. As the same app is used on multiple platforms, it does not matter if it's one device or ten, your file is accessible from the cloud and your AppGameKit app can retrieve it from anywhere at any time. There are one or two threads on how to create a server side app with AppGameKit, but if there is more call for it we will write a tutorial/video on the subject as it's a great mechanism for all sorts of cool app ideas (if you have access to a server plugged into the internet).
I drink tea, and in my spare time I write software.