No, you won't need multiple installs.
Maybe the "subsites" module (http://www.silverstripe.org/subsites-module) provides what you need.
Otherwise you could use the same code-base for multiple websites by using symlinked folders. I do this for local development, where I have one SilverStripe code-base and multiple websites that are using this code. Makes updating the CMS very easy across multiple sites.
This thread contains some information that might help you: http://silverstripe.org/general-questions/show/251650