Connect to multiple AP with one Wifi adapter under Linux/FreeBSD?

Under FreeBSD (pfSense is based on FreeBSD and has nothing to do with Linux) you can create multiple wlan interfaces based on the physical interface, then connect them to various APs. For instance, if you have an Atheros card in your computer (and it isn't renamed):

ifconfig wlan0 create wlandev ath0
ifconfig wlan bssid NetworkA [wep/wpa/etc info]
dhclient wlan0 (assuming DHCP)

if you wanted another connection just spin up another:

ifconfig wlan1 create wlandev ath0
ifconfig wlan bssid NetworkB [wep/wpa/etc info]
dhclient wlan1 (assuming DHCP)

If the two networks are on different channels you'll see a reduced throughput, noticeably so if you're actively using both.