Paul,
You are correct that the jetty module system issetup to support only a single instance of each type of connector.
To have your setup, you will need to at least right an XML file and perhaps a custom module to make it easy to enable.
You need to copy/paste/edit the files:
${jetty.home}/etc/jetty-ssl.xml
 ${jetty.home}/etc/jetty-https.xml
 ${jetty.home}/etc/jetty-proxy-protocol-ssl.xml
to ${jetty.base} and call them jetty-sslB.xml, jetty-httpsB.xml & jetty-proxy-protocol-sslB.xml.  Edit them so that they refer to the id `sslConnectorB`
You then can create a custom module in ${jetty.base}/modules/httpsB.mod with contents like:
[description]
Adds alternate HTTPS  Connector
[depend]
ssl
[xml] 
etc/jetty-sslB.xml 
etc/jetty-httpsB.xml 
etc/jetty-proxy-protocol-sslB.xml
You can replace `B` everywhere with something more descriptive... I tried `2` but that is confusing with `http2`.  You could also merge these files into a single XML if you liked.
So, in summary, you need your own XML to create the second SSL connector with the different configuration.
cheers