I'd like to help you with this problem, but I'm not clear as to what you're trying to accomplish here. I think the confusion is that you're using sub domain to mean two different things. Your first reference to sub domain might not mean business.abc.com but instead mean that your existing site has two main sections. If that's the case, how are the sections currently seperated? Is it based on sub folders (ie. abc.com/residential and abc.com/business) or is there some other method for seperation?
The solution, if the end result would be business.abc.com and residential.abc.com will depend on how the site is currently developed.