Go to settings-->Miscellaneous and scroll to the bottom of the menu. You should see an option for "Sync reading positions via Dropbox" or "Sync reading positions via Google Drive" that you can check off. You'll have to do this on both devices. I've only tried the Dropbox option; when you initially select it, you'll be taken to the browser to confirm to Dropbox that you are allowing Moon+ Reader to access your Dropbox account.