[DutchDaemon]

I've never done a 'dual dhcpd' setup, but I'd assume something like this could work:

dhcpd_ifaces="xl1 xl2"

In dhcpd.conf:

subnet 10.100.0.0 netmask 255.255.255.0 {
  range [...];
  option [...];
  option [...];
  default-lease-time [...];
}

subnet 172.16.0.0 netmask 255.255.255.0 {
  range [...];
  option [...];
  option [...];
  default-lease-time [...];
}

In other words: declare all options in separate subnet containers. This is all under the assumption that dhcpd will know which interface a dhcp request is coming in on and which subnet to associate with that request.