That should work just fine. The only thing to be aware of is if you use any sort of controller bases authentication, webauth or wispr, each time a client roams between the two, they will need to re-auth. And the other thing is that each system will see the other system's AP as rogues, but beyond that it should work.
802.1x roaming would also be an issue as the AP's are the authenticator and its not straight forward for the clients, as they think they have a DHCP lease, but the AP is not aware that they have connected to the network; older ZD1000 will not support 802.11r so no BSS transition.
For these clients they need to manually turn their card on and off and reauthenticate, as you also dont have the "Force DHCP" option available on the ZD1000.
Sid and Sean, Thanks for the answers. The client will not be doing 802.1x or anything fancy. This would be for a convention center so the clients will connect with wpa passwords and just do internet access..
Based on your answers it looks like the solution to have both ZDs would work (as long as he does not do anything fancy..)