It has to be in there at boot, but that's what I'm told works. I think people just leave the USB inserted and have both active, and whichever one can connect does. Are you trying to connect to two different available networks, or have two networks so you can change locations and still connect, or have one AP for on the road / client mode for at home? I think the last case there is the most common.
Note that the Wifi configuration when you download the firmware image, and the wifi page under LinkMeter in the webui are only for single networks. To set up multiple you need to use the Network -> Wireless section in the standard LuCI configuration to manage them.