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I'd say do one site in two areas:
expertise1.company.com - content and meta data related to area of expertise #1
expertise2.company.com - content and meta data related to area of expertise #2
www.company.com - small (2-3 pages) section with the "about our company" data, with prominent links to each 'area of expertise'
-or-
Do three totally separate sites:
www.expertise1.com, www.expertise2.com, www.company.com (pretty self explanatory)
Mirror sites lend you no opportunity for maximizing your optimization for your site and in fact often hinder your chances.
typophile - Mivox's posting is right on. Your comments about meta content suggest that you share a basic and common misconception about what meta tags can do. Meta tags are not magic bullets that assure good ranking. Meta content and page content need to be related. If the content isn't there on the page, including it in meta tags generally isn't going to help.
Additionally, page focus is an important component of optimization... and it's very unlikely that the same page content will be focussed on two different sets of search targets.
And yes, dupe content on mirror sites will get you in trouble.
The content is there,we are combining these two areas of expertise in synergy so the content will contantly refer to both areas on most pages. The problem is that this synergy is very new so it is very unlikeley that a user will combine search terms that refer to both, thus my idea about mirrored sites. Anyone have a better idea?