I am trying to set my WHR-HP-AG108 to use b/g mode on ath0 but I'm half-there. The interface seems to be up but it's not broadcasting the ssid and you cannot associate with it.

I have tried following without success (wifi1 and ath1 are fine though):

config wifi-device  wifi0
        option type     atheros
        option channel  1   #(normally this is 149 for a-only operation, I've tried 1 through 11 for bg but nothing works)
        option agmode  11bg #(normally I don't use this but it doesn't help, either)
        #option agmode 11g #(this doesn't work either)
        #neither does without agmode, using only channel 1.
config wifi-iface
        option device   wifi0
        option network  wlana
        option mode     ap
        option ssid     "test.ssid.a"

config wifi-device  wifi1
        option type     atheros
        option channel  6
config wifi-iface
        option device   wifi1
        option network  wlanb
        option mode     ap
        option ssid     "test.ssid.b"

Any input from someone experienced in madwifi and a/b/g interfaces greatly appreciated!

Let me rephrase my question -
Anyone have experience with Atheros 5312 WiSoC which contains one 5112 and one 2112 rf unit.
I have found one thread on here that discusses problems with a related device and says that this unit only has 11a on the first wireless device and b/g on second, which, according to http://www.atheros.com/pt/bulletins/AR5002AP-2X.pdf (Atheros documentation on this device), is not true. They say that the AR5112 (the first wireless device of 5312 chipset) supports frequencies 2.3-2.5 and 4.9-5.8 and AR2112 (the b/g unit) 2.3-2.5. This chipset is a dual-radio setup, not a single a/b/g that has somehow been made visible through madwifi as two separate devices.