you can see an example in http://www.ehcp.net/?q=node/612

that example would work on local server.
on your case, for ex, ehcp is running on server A
but you want to connect from B, and order ehcp to setup a domain,

then, you need to connect from B to A, and execute that code on B's mysql.
to do that, you need small modification in B's code (connecttodb) .

* copy ehcp files onto B,
* modify connecttodb, to reflect ip of A
* call $app->addDomainDirect

Maybe, in future we may develop a more easy way. you may need some fine tuning. we may discuss here.

About double dns server:

Unfortunately, there is no direct and easy way of doing that, However,
it is not impossible.
To have two dns servers, one for master, other slave, you need some
work, some settings and some testing.
lets say, you have two servers, one is master (server B), the other is
slave dns (server A) and A wants to connect to B to update its
domains.

Here are initial steps:
1- install ehcp on both machine,
2- as described in http://ehcp.net/?q=node/827#comment-1304 , prepare
for mysql connection from A to B. To test your remote mysql working,
setup a domain in A, you should be able to see domain in other
machine. if you get some errors, let me know.

we have some more steps after you complete these 2 step.
Let me know after you complete these steps.
It would be useful that I have access to your panels, and servers, so
that I can check what is going.

api resources-wiki-http://www.ehcp.net/wiki/index.php/API_Resource's