Hi bez
Assuming you run an Apache 2 webserver I would suggest you set up two <VirtualHost> directives:
<VirtualHost *:81>
</VirtualHost>
<VirtualHost *:82>
</VirtualHost>
We would do it that way.
What we've actually been doing for quite some time is running several Dokuwikis on port 80 (HTTP) AND port 443 (HTTPS) with some redirection 80 >> 443 active.