Question : Creating Secure Public and Secure Private Networks (Hotspot) - Linksys WRT54G Hotspot

Hello!
How can I setup secure free wireless for my client's customers and maintain a separate private network for the client?
My idea was having 2 WRT54G routers Public/Private
WLLAN - Private - WAN > PORT1 - Public > Internet
                |                                    |
              LAN                              WLLAN

With "AP Isolation: ON" at Public OFF at Private (To deny / allow inter-computer communication respectively)
Now I just want to make sure people on Public cannot access Private in any way, while still allowing internet traffic only through Public from Private.
Does the above work?

http://www.experts-exchange.com/Networking/Q_21474533.html
The above answer states "connect the two wireless routers with a crossover from LAN port to LAN port" but I don't believe private and public should be on the same network.

Answer : Creating Secure Public and Secure Private Networks (Hotspot) - Linksys WRT54G Hotspot

Your method should work. I assume you are using NAT both times, wich is pretty secure when it comes to keeping people out of your internal network. It can be a drawback to use nat twice as sometimes NAT might get confused by this, but this depends on the NAT implementation(s) used. It will probably work.

If you have an DMZ interface, placing an wifi router at the dmz port of another router, allows you to controll access from the DMZ to either the internal or the external network(s).

You don't NEED the DMZ, but if available, i would rather put one in DMZ then NAT twice.

Another reason for the DMZ could have been the lack of enough external ip's, but as i understand it now, the internet router is actually the same device as your WIFI router. If this is the case (most likeley), then the wifi is already on the internet router, and you do not need an extra (DMZ) interface.