CEO + Founder of MIIM Designs LLC, inspiring solutions with social impact, focusing on community, culture, sustainability, and economical design solutions. Undergraduate Adviser for Harvard University - History of Art and Architecture. Adviser at Harvard Social Innovation (HKS) and Harvard innovation Lab. Expert in Social-Architectural Enterprise in developing countries - India, Ghana, Sudan, Afghanistan and etc. Pluralistic Architecture, Women in Architecture, Women in Start-ups / Small Business.
It is best to create 2 different sites and link them together - advertising the other on their own webpage.
As everyone has mentioned, the SEO, is the main key player here. Because such search engines like GOOGLE and Google Ad Words are used for marketing purposes, you would gain more benefit from having 2 websites.
Take a look at these 2 website owned by the same people:
http://www.hyarchitecture.com/
http://www.mystudio.us/
The layout is exactly the same and the "SWITCH TO:" is in the same place. The services are different, but the user will know that they are same people, but different services.
I hope this helps. If you need anymore examples or ideas, feel free to contact me.
Maryam