Roaming guest wireless


I have two omnias. One that acts as my main border router with NAT 192.168.21.x wired and wireless lan over 2 and 5GHz.
I also have a manually configed guest wireless on the 2GHz card. The guest is separated and have range 192.168.16.x

On my second internal omnia is configed as AP only and have its wan and wifi bridged.
I use the second Omnia for some lab work and experimenting without effecting the main one.
Anyway - I would like to set up a 2GHz guest wifi on the internal AP to be able to roam better around the larger area.

But how should I config the guest net to use the dhcp and DNS from the border omnia?