I heard it suggested on the radio that assuming that they are allowed to take over Yahoo, they'll likely keep both sites up.
as it is both Yahoo ad MSN are a trusted brand in households, I know plenty of people who'd still go to yahoo as a search engine rather than google, just out of habbit, upsetting peoples habbits might actually drive them to use google instead,
the cost of keeping two sites with twice as many ads on display (meaning the ads on both sites, not an increase in ads), would probably lead to more revenue than cutting back to one site and potentially loosing ad impressions.
of course they could link to a single portal that's accessible from both names, but I can't really see microsoft changing a lot to do with Yahoo. just skimming a tidy profit from the company really.