There are two main types of such search engines. The first type is one that combines multiple search engines; this is called a meta search engine. Examples of meta search engines are:...
There are differen kinds of multiple search engines: Webcrawler.com, metacrawler.com, dogpile.com are searching both Google, yahoo, bing at the same time, and give results from both engines....
I had a similar problem and tried for months to sort it out. In the end I used a company to help. They are called ClydeStan. http://www.clydestan.com There is lots of info on their site how they do...