Across accounts. I believe it auto updates information on the same account. So Acct 1 will always have all the valid information for all alts {that have been logged in} on that account. Same with the main on Acct2 and so on.

To get the information from Acct 2 on to Acct 1, you have to share it over. I do it once every few days when I'm taking a flight or waiting on daily resets. Unless you have huge and constant turnovers of items, there's not really much need to do it all the time.