One Webhost and 2 Domains? wtf will happen?

ok, a simple question guys.

I've registered a domain and webhost with Hostmonster.
So the url is: www.123.com

So let's say I want to have 2 domains in hostmonster.
So I buy a domain from namecheap and link to hostmonster's name server.

Then I will create a folder and name it "site2" in my public_html.
So the url will be like this: www.123.com/site2
And now I assign the folder to namecheap's domain, which will be www.456.com

Now the question is... won't Google crawl and index the site2 folder as www.123.com's contents?
What are some moves i can take to disallow that from happening?

BlackSeng said:

Then I will create a folder and name it "site2" in my public_html.
So the url will be like this: http://www.123.com/site2
And now I assign the folder to namecheap's domain, which will be http://www.456.com

Click to expand...

It's easier to add the domain in cpanel, then change the nameservers.

Thanks guys...