Two-way synchronisation means information flows in both directions. If a customer's phone number is updated in the accounts package, the customer system receives the change, and the reverse also holds true. Neither is merely a copy of the other. Both are treated as live, and each keeps the other informed.
It is more demanding than one-way copying, because the connection must handle conflicts, such as two people editing the same record at once, and decide which change wins. Where possible, many businesses prefer one system to own each fact. Two-way sync suits cases where both systems genuinely need to be edited.