The easiest way to do it is with the "Reading List" Plugin. I have two ereaders, a phone and two tablets. All least one (iPad, and I'm in my way with the CC for iPad) are synchronized automatically in two ways:
- Reading List Plugin: It send to the device all the calibre books which are not in the device. It populates automatically when a device is connected.
- Smart Eject Plugin: Prior to eject the device, it detects if there are duplicated books in the device or if there are books in the device which are not in the library