Is there a tool to “diff” search engines?

Question

Basically something that gives results from search engine A that are not results of search engine B. Or maybe results from A that are ranked much much lower on B.

Wildgoose · Accepted Answer

Many years ago I used something called "dogpile" that simultaneously queried several (around 10 I think) search engines and then returned the results retrieved.
Not quite what you are asking, but similar.
Perhaps more interesting would be a search engine that simultaneously checked in different legal areas and used the differences to identify where people or organisations have tried to use the law to suppress information.
Here in the UK we used to have a situation in which Scottish versions of newspapers sometimes contained news that had been suppressed here in England. (Our respective countries have different legal systems and sometimes there would be legal rulings in one or the other, but not both).