I want to tell step by step, since this is some complex thing,

first, a separate mysql server: (this is mostly from ehcp gui)
normally, a separate mysql server is done like this:
scenario:
server1: mysql
server2: other hosting functions, webserver, dns, etc..

to achive this:
* install ehcp on both server
* on server 2, login to ehcp gui, options, advanced settings/options, check multiserver, submit, then,
* click add server
* you will see a page with server properties.. (only mysql ser can be configured with this interface), attached image is this,
* in input fileds enter:
-> serverip: the ip of mysql server, internal or external ip.. this ip should be reachable by this server.
-> servertype: choose mysql server,
-> server option: click whichever you want, i suggest "always" for start, otherwise, each time user tries to add a db, a selection of mysql server will be presented,
-> if optional: if above option is not "always", this means is this server is default mysql server, to be presented to user,
-> mysql pass: enter mysql root pass
-> default mysql user hostname: you should write ip of webserver here, in this scenario, ip of server2, in next scenario, ip of server 1 which will contain webserver, read explanation right to it to understand..
(This is host of mysql user, to connect from,
You should write hostname/ip of your webserver here.. Otherwise, webserver cannot connect to your mysql server.)

* click submit. this way, you identified your mysql server at server1 to ehcp on server 2. From now on, you should use ehcp on server2.
* (XX) go to server 1 (on which mysql server resides) go to phpmyadmin, click on privileges, click on edit on right of root user, goto "Change Login Information / Copy User", write ip of server 2 to host input, keep the old one, then click go; this way, you let server2 to connect to mysql on server 1. this way, server 2 will be able to manage server1's mysql. understand ?

(In future, i think to simplify these things more and more, just like scenarios I wrote on above posts)

--------

Up to now, I only told about separate mysql server. now, we are going to add webserver capabilities to server 1:
I want to note that, adding a separate webserver or ftp is not directly supported, or easily done in ehcp, So, we will do many manual tasks..
if we add webserver to server1, we also need to give it ftp duty. Because files should be put on there for webserver.

first ftp: (on server 1)
open file: /etc/pam.d/vsftpd
edit part: host=localhost, replace localhost to ip of server2, with same user/pass as server2 (you should use all same passes in both ehcp installs in both servers, to simplify things)
restart vsftpd: /etc/init.d/vsftpd restart

for webserver:
what we need: ehcp handles all apache configuration files. So, to let apache function by information on server 2, we need to modify ehcp's config on server1 to read its database from server2, not itself.
so,
on server1: go to /var/www/vhosts/ehcp
edit config.php
$dbhost=ip of server2;
other parts same, if you use same pass in server2

now, go to phpmyadmin in server 2, do same as (XX) above, to let server1 to connect to server2 through mysql. (so, both servers see each other through mysql)
restart ehcp on server1: /etc/init.d/ehcp restart

this way:
ftp in server1 will read users from server 2,
webserver in server1 will read domains from server 2 and setup dirs and configs related to that,

server 2's ehcp gui will be used to manage things.
server 2 will do dns, postfix, etc.. as usual...

I know that this is quite complex, but unfortunately, there is no more easy way yet, as far as I know... Maybe somebody else may find some more easy way..

In future, i think to make things like scnearios, give a server a functionality, build server plans, and make them work out of box... I hop i will do it as I have time..

see you,

image at: http://www.ehcp.net/sites/default/files/ehcp_multiserver.png