It's not fair to combine the two since there will be some cross searching and Yahoo users who will not use Bing but like Google, and all that. When the actual merger takes place, I'd bet the combined total actually goes down by a few points.
Also, for what it's worth, I now use Bing as my primary search engine and find it produces highly relevant results. I attribute this to performing tightly targeted search queries. I feel that Google is still better for serendipitous (exploring type) searches.
