I think merging and synchronizing are two different steps. One solution is to use a common network based address book such as Google Contacts. This requires that you sell your soul to Google, use the gContactSync addon, and live with some idiosyncrasies due to some mismatch between Thunderbird's and Google's ways of handling address formats. However this is the best I have found for sharing an address book between Thunderbird and Android devices such as a phone. Between two instances of Thunderbird, I use the Address Book Synchronizer addon and do the share via Dropbox. There are other sharing options: ftp, imap, network shared folder. It's not quite synchronizing, because it's not real time, and it's safe only if you use only one device at a time. So it works for me for sharing an address book between a computer at work and another at home, because I'll never use both at the same time. As to merging, you could move the contents of existing address books into the new shared address book and use a duplicate contacts manager addon to reconcile repetitions.