The install of ISPConfig on the second server went OK.
And I made sure to use hostnames for every question it asked.
I had no problems setting it up, and getting things to whirl around.
(No MySQL connection problems).
I can still access the main server from mysql command on the second server.
I did not however edit the hosts file, and will have a read in the multi servers guide to see what it actually wants me to input.
But both servers can ping eachother via internet, so the name must resolve somewhere?