It is supported, but there's just no space for dual-ddns in the New Site dialogs. But it is easy to add after you created your site.
Go into your site's config folder, for example config/mysiteexample.dynip.org
In there, find the folder 0_site/settings
In there, edit the text file ddns.txt (if you are running Windows older than 10, right-click and open in wordpad so the line-endings are correct)
In this file, you can place something like the following example lines:
update_url_4:http://blablablablab.bla/updateip4
update_url_6:http://blablablablab.bla/updateip6
ipcheck_url_4:http://checkipblablablab.bla/check4
ipcheck_url_6:http://checkipblablablab.bla/check6
Don't leave any blank lines in this file.
If you see any old leftover update_url: or ipcheck_url: lines you can clean those out.
It's documented on
https://help.supersimpleserver.com/config near the bottom in the specialized protocols section, but if you have any trouble please feel free to reply in here.