If I understand what you are thinking about, it might be just as possible/effective to also do it via two separate web sites that both point to the same smugmug account. Not as a custom domain, but as custom navigation from there. Allow me to illustrate
Your have a domain called "coolstuff.com" and another called "otherstuff.com" you also have a smugmug custom domain that is "photos.stuff.com"
On the smugmug account you could set up two categories like Andy suggested, one for "Cool" and the other for "Other"
On the site coolstuff.com you place a landing page, and perhaps an about or other content. Perhaps a blog. You do the same thing on otherstuff.com.
You create a link to galleries or portfolio or images or whatever you want to call it. On the coolstuff site you point to "photos.stuff.com/cool" and the other site you point to "photos.stuff.com/other".
You can do some SEO on the microsites for each domain that might assist with offsetting some of the disadvantages of not having discrete sites for SEO with SmugMug.
However having tried to split up my site, one area private & one area public, after about a year I decided that it was easier to just set up two accounts. Less worrying that customizing one section would break another section.