two different spiders, two different presences on your site:

1- Googlebot: a single bot which visit each link of your site page by page
2- Yahoobot: a pyramidal bot which will call one friend per new link found on a page

Google will appear once, on a long period because it visit the site on its own... if you see multiple google bots, they usually are driven by references (too much pages to see, would be too slow if a single bot)

Yahoo bot will have so much links to visit that your sessions are overloaded like if there is really millions of visitors, but it's just a cascade of different IP used by a single engine.