Hi,
you have two easy choices:
1. (my prefered)Have you site hosted on an ISP which allows serveral domains to be parked, but only one site. This means your visitor goes to
www.site1.com and gets your site, or
www.site2.com, getting the same site, none the wiser that you have them both there. (my ISP
http://www.freevirtualservers.com/ does this)
2. Have a redirection on one page so that your visitor goes to
www.site1.com but then automatically gets forwarded to site2. The problem with this is that the URL changes and can confuse the average surfer.
HTH
Sparky