The two transactions that require special surveillance of financial institutions under the Money Laundering (Prohibition) Act are:

1. Transactions involving large sums of money exceeding the threshold prescribed by law.
2. Suspicious transactions, including those that are unusual, complex, or lack a clear economic or legal purpose.

Explanation:
Under the Money Laundering (Prohibition) Act, financial institutions are mandated to keep special surveillance on transactions that exceed specific monetary limits or appear suspicious. These measures are in place to detect and prevent money laundering activities and ensure compliance with regulatory requirements.