The sync collections setting doesn't copy items between devices, all it does is keep the books/PDFs that you already have on those devices in-line i.e. if you have a book already on multiple devices/computers and you move it into a collection on one device then it will be moved into the same collection on the others devices/computers that it's on.
You will need to connect your iPad to your Mac's iTunes and do File > Devices > Transfer Purchases to copy them over to your Mac (that should copy PDFs and/or epubs that are in the iBooks app on your iPad, not just actual ibooks), and then in the iBooks app on your Mac do File > Move Books From iTunes.
You can then sync them to your other device by connecting it to your Mac's iTunes, and the books/PDFs that you have in the iBooks app on your Mac should show on the device's Books tab in iTunes for being available for syncing to it.
e.g. connect the other device and select it on the left-hand sidebar of your Mac's iTunes (you can enable the sidebar via option-command-S), and on the right-hand side of the iTunes screen use the Books tab to select and sync them