One domain will be your primary domain. It should automatically be available at http://www.yoursite.com, and the files are in /home/ACCOUNT/public_html/. If you purchased additional domain(s), you would probablly have to explicitly add them as "add-on domains". Your control panel (cPanel or LPCP) has an entry to do this. You might have to use the domain name management system to "point" the second domain to the same LP nameserver as the primary, if it hasn't already been done for you. At this point you will have http://www.yoursecondsite.com available, and the files will be in /home/ACCOUNT/public_html/yoursecondsite/. Don't forget that it may take a day or two for any change to domain name management to be recognized by your browser (the change has to spread across the DNS system).

For SEO purposes, I would suggest creating two separate and unique websites that have links to each other. I read that having multiple domains pointing to the same destination can actually be bad for SEO.

Also it wouldn't hurt to have two websites geared toward the same nitch, this way you have twice a better chance of generating traffic/business