Seems like a common use case to have if you allow the primary domain to have subdomains. Changing the .primarydomain.com text to a drop down of the primary and secondary domains it would work perfectly.
Also of interest, it would be nice to be able to change the primary domain for the account as well. Likely from nodeworx, but making it a feature you could enable for siteworx users wouldn’t be bad either. As it is, it’s a lot of hassle to create new accounts and switch everything over for the occasional user that gets a whole new domain.