Multiple Domains Pointing to the Same IP Address

How Does Yahoo Treat Multiple Domains That Point to Same IP Address?

[dmarbiz]

I have a client that has two domains (Company.com and CompanyName.com) pointing to the same IP address. The content on both sites is identical. However, I have found that the Search Engines are treating them very differently...
Google has a total of 475 URLs indexed for Company.com - but only 15 URLs indexed for CompanyName.com. And, Yahoo is showing a total of 412 URLs indexed for Company.com with only 15 URLs indexed for CompanyName.com. Why is there such a disparity here?

I doubt this client will be choosing the 301 redirect option at this time... They want the multiple domains to have more paths leading into their site. And, they realize that using the 301 redirect for the CompanyName.com domain would eventually lead to none of the CompanyName.com pages being indexed by Google. However, this probably would not make a difference with Yahoo as they are still having problems with 301 redirects.

Any advice I could get on this topic would be appreciated...
How is this type of setup (multiple domains pointing to same IP address) indexed by the SE's?
Will this type of setup have an adverse effect on the SEO efforts for this client?
Are the pages from the CompanyName.com site being treated as duplicates and thereby penalized by Yahoo?