Thanks Simon.
As developer of PiNet, I want to point out, PiNet does not include a DHCP server, nor does it include any easy way from the PiNet control panel of adding one. If you want to add a DHCP server, it very much leaves you on your own. The thinking being, if you are able to install one and set it up, chances are you probably know that running 2 DHCP servers on the same network isn't a particularly great idea..
I am not sure why you need a second DHCP server? The DHCP server does not manage who has access to the internet, although internet access may depend on which range you are put on I guess? Either way, the main DHCP server should be able to be configured to simply hand out addresses to the MAC addresses of the Pis and put them on a different range?
PiNet itself is set up to basically be passive on a network. The address of the server (which should have a DHCP reservation) is included on the SD cards themselves with the boot files.
So to reiterate, PiNet does not include (or make it easy to install) a DHCP server and it is not recommended to do so.