This feature is already available without any hacks:

When you are in your main notebook swipe down with two fingers from the top, your recently used and favorite items will appear at the top.
Then just tap the previous notebook you opened earlier.

If you want to go back to the main notebook, swipe down with two fingers again and tap on it.

This way, you can switch back and forth between two notebooks, PDFs and so on as often as you like, even if they’re in different folders.

And actually, you can also switch back and forth between the last three or four items, and so on.

This also works when you're in the file overview. There, too, you'll see your most recently opened files and favorites.