Federated Search engines take a user's search and sends it to multiple search engines, then combins the results back together.  This approach is sometimes preferred as it doesn't require any single engine to index all of the content.

Key federated search requirements in the enterprise

• Flexibility in combining results
• Maintaining & Mapping Security Credentials
• Advanced Duplicate Detection and Removal
• Combining results list and page Navigators
• Translating into different search syntaxes

See the detailed list.