In addition: don't forget to set eth0 (connected to your internet/modem) as external.
If you connect eth1 to a switch and your LAN port of the AP to that switch, (or alternatively connect the LAN port of the AP directly to eth1) you can use Zentyal DHCP on eth1 to distribute IP addresses.
As Escorpiom pointed out, it is important to have different subnets on eth0 and eth1!
If you have further questions, just ask. Give as much details of your configuration when asking questions, so we can give you the best answer possible.