A search engine that allows you to search several search engines simultlaneously is called a?

That is not exactly true. A "meta search engine" pays importance only to the meta tags and meta discriptions of the HTML code.

A search engine that searches many search engines is a mashed search engine, because it mashes the api of dfferent search engines.

Examples for meta search engines are: http://metaspider.com/ or http://megaspider.com/ (very old)

Examples for mashed search engines are: http://dogpile.com/ or http://avasearch.com/