I recommend that you set eth1 to a static address on a different subnet from the one used by eth0. You could use 192.168.2.1, for example. The 255.255.255.0 mask is fine.
Then, within Zentyal's DHCP settings for eth1 you want to create a range of addresses that Zentyal can assign to the computers on your network. Still using 192.168.2.x as an example, you could create a range of 50 addresses, from 192.168.2.50 to 192.168.2.99. Zentyal will then use addresses from that 50 address range to assign addresses to your network computers. The actual range you create can have different starting and ending addresses, but you need to create a range.
Once that's done, please let us know how things are working.