Actually no, the synchronize will only synchronize with the current account that you are logged in with. Some people log into multiple accounts on their computer and want to keep the friends separate because they are completely different channels.
In order to do what you are looking for, you will have to create a new group on your own called "All Friends All Channels" (or something like that), then gather all of the friends from each account into that group.
You can always suggest that as a feature at
http://tubetoolbox.uservoice.com but it would be pretty complex so I don't see it happening anytime soon...