As the title suggests, two nations that want to use the same root domain cannot do so, even if they want to use different subdomains (e.g. engage.example.com and www.example.com).
In the past, the only limitation here was the email authentication. In other words, the second nation could add the other subdomain as a non-NB-managed domain and get the site up and running at that URL, but the email broadcaster would not be able to use that domain, as the custom whitelabel records were specific to the other nation.
However, with the new domains UI change, the second nation cannot even use the domain for a website now, as adding that domain requires adding the root domain first and then defining the subdomain in the "Sites" tab of that domain's settings.
This is something we will consider but does not have a timeline at the moment.
However we are paying close attention to the most popular suggestions so please share and encourage others to follow and comment!