In my experience no, I have ranked at no2 for a very competative phrase, pretty much by accident if im honest, on yahoo, and for the same phrase on the big G im on page 21 i think.
Its a good source of traffic for me, but it isnt mirrored in Google
if you are ranking on Yahoo, then your on page is pretty much good to go. You now need to slowly link build across the site to rank in Google. How old is the domain?
Yahoo and Google use different systems, though there is a lot of cross over.
Frankly, unless Yahoo is specifically valuable to your market, I'd focus on Google first - a site that is strong in Google is more likely to do well in Yahoo as well, but the reverse is less likely to be true.