Users often write in that they have multiple domains they want to use with their SmugMug site. It's often something like yourdomain.com and all the .net, .biz, or .org variations.
It also can be for various spellings or business areas—example.com, examplephotos.com, and examplephotography.com are common.
First the bad news: SmugMug only supports one domain name at a time. The good news is there are ways to make this work.
Before you do anything else... Set up your primary domain using the standard instructions before attempting to work on any secondary domains.
For most domain hosts, all you need to do is forward the second domain to your primary domain. Don't use domain masking, as it will cause problems.
Other methods that you can research on your domain host's help pages are building an HTML Redirect page or an .htaccess file on your domain host's servers.