do you really need two sites or you need a site with directory structure?

Options are:

- create a site with no path(which will be your root site then you can have pages under this site "abc" and "def" - if you don't need abc and def as paths

- keep your existing sites but again create a new site with no path and redirect to one of these sites when sme one browses the root site via the root site active home page 

I am a site newbie, so pardon if answers to my questions seem to be really obvious.

I still need two sites. those 2 sites cater to 2 different audiences. But I dont want the user to be presented with a  under construction page when he goes to root. 

Is creating a new site is the only option

Can I create a Visual force page and 2 links to each subsites. If i create a VF page, in which sub site should it reside

Also how would I say the newly created VF page is what should be displayed when a user traverses to the root 

Thanks 

the new page with 2 links (alternatively page can redirect to one of the sites you have by default) would reside in the new site you would create (/ site with no path)

alternatively you can mask your site default urls with a custom url like (www.abc.com would mask xx.force.com/abc, and www.def.com would mask xxx.force.com/def sites) so you won;'t see this issue

Thanks Bullent for the detailed reply. 

I like your second option, but I can't seem to find where the site default URLS would be.

Is this the setting you are referring to

Default Web Address

when I click on setup-> sites-> and click on the site label? 

The default web site does have an entry like companyname.force.com/abc and companyname.force.com/def already