Ask your ISP to setup an Alias for the second domain name: site2.com CNAME
www.site1.com. This will make site2.com resolve to
www.site1.com.
The site2.com address will stay in the address bar if you do it like this.
If you want site1.com to show in the address bar (i.e type in site2.com and then it changes to site1.com) and you registered your site2.com domain name with a decent registrar you can just setup a forward (usually free) that forwards site2.com to
www.site1.com.