For more information on your device, select it on the Support main page.
Two phonebooks can be used for two different users. For privacy, only one phonebook is available for a specific user (i.e. phone) and the phonebooks can neither be merged, nor be used at the same time by toggling between them.
Once downloaded, the phonebook is usable. The next time one of the two device is connected to the car kit, the download runs in the background and the updated contact list is available when the download is finished.