Oh, it's not necessary to wait for 15 minutes.
I think 2 or 3 minutes, at most, should be enough. Normally,
seconds should be enough, if you're on a WiFi network. You could open a desktop computer/notebook of yours and as soon as you see the sync file arrive in Dropbox there, you can try opening the book on the 2nd device. (Which reminds me of another
feature request.)
Please, make another sync test, but before you do so, go to your 2nd device and flip to the
beginning of the book. That way, you will be certain to avoid that "You are already at the furthest location" message there -- if syncing is working for you at all.
I think you may have been in the correct place from your previous sync attempts, which is why the test failed to show if sync is working for you or not.