I use option 43 and 60 in DHCP server.
The only weird problem, is that my AP on the deifferent subnet gets an IP from the other subnet!
This is causing a weird slow conenction. Data traffic seams to be doing a loop...what I mean is when a wireless client in the "b" subnet does a request for data in the "b" subnet, it loops through the "a" subnet causing very slow connection/transfer. I haven't looked much into this yet, but will have to soon.
Btw, these two subnets are via WAN (100mbps).