The companies may come out with a press conference when both the boards have finalised the terms. Nevertheless before the merger take place, the consent of the shareholders is required.

Joseph is right, the common investors can find out only after the board has approved. Any major M&A decision is finally subject to shareholder approval, but rarely has there been a scenario were the board approves the merger and the shrehodlers dont. The board is supposed to represent the interests of the shareholders.

The reason this news is kept secret is because otherwise the share prices of the public companies may be unfairly affected.