Viewing both sets of results in different windows is a bit tedious, and a clever Norwegian developer named Asgeir S. Nilsen has made the task easier at a site called Twingine.
The Twingine site contains a blank search form where you can type any search query. When you click Search, the site brings up the results pages for that query from both Yahoo and Google, side by side. For fairness, Twingine randomly changes the sides that Google and Yahoo come up on, so people who prefer one side of the screen over the other won't be biased.
Paul Bausch's book Yahoo! Hacks mentions Twingine in Hack #9. Take a look at Yahoo hacks - Insider Secrets - CNET.com, tip #3: