Perhaps I didn't explain very clearly either.
If you're happy to load the book into Calibre twice then you can do what you want by converting one for 6" and one for 5". You will, of course, have 2 lines displayed in the Calibre window for every book, which some would consider messy, but if you can live with it that's all that matters.
If you only want to store the book once then each time you create an epub it will overwrite the previous one. So you would have to create 5" epub and download to 5" reader. Then create 6" epub and download to 6" reader (or vice versa).
The original point I was trying to make is that you could try, just as an experiment, creating the 5" epub and downloading to the 6" reader to see if it was readable. If it was acceptable (maybe not perfect) you could save yourself a lot of future effort and keep a tidier library.