Mattias mentioned today, how to create virtual interfaces. Since i didn't find information elsewhere, i will share his hints and my thoughts here.
1) You need some recent kernel, maybe the rt2x00.git tree.
2) You need a supported hardware... rt73usb works fine, rt61 should, too.
Installing the "iw" tool:
3a) You need libnl and libnl-dev. In Debian, a "apt-get install libnl-dev libnl" should be ok, on other systems, refer to their method of installation or download, compile and install the libnl from git://git.kernel.org/pub/scm/libs/netlink/libnl.git
3b) pull the "iw" tool from the git repository and compile it:
# cg clone http://git.sipsolutions.net/iw.git
# cd iw
# cp defconfig .config
If you like to use the "iw" tool more often, you can install it to /sbin (as root):
# cp iw /sbin/iw
4) Your wireless card should be plugged in, there should already be a single interface, say "wlan0". Now you can create a new interface with the iw tool (replace "wlan1" with a name, you like):
# iw dev wmaster0 interface add wlan1 type managed
You should now be able to see the interface with "iwconfig". Configuration and association works just like with any other interface, but be aware, that the hardware currently does not support connection to different APs or mixed AP/Client mode. Multiple interfaces are thus only helpful for "monitor" interfaces and sniffing tools.